GSB Coverage Calculator(Granular Sub-Base Tonnes & Brass)

Calculate compacted GSB coverage.

Calculate GSB tonnes or brass for compacted sub-base area and depth with compaction allowance.

🕒 Last updated: June 20, 2026

Inputs

Aggregate Type & Use

Bulk Density (kg/m³)

ℹ️Auto-filled by material. Replace with supplier-tested density when available.

Target Area

Depth / Thickness

Bag Size for Results

▾

Quantity to Order (incl. 6% wastage)

30.27 tonnes

30,274 kg

12% compaction allowance included

Number of Bags

606 bags

36 × 850kg bulk bags

Rounded up to complete bags

Compaction applied: results use finished layer dimensions. The loose material allowance of 12% is included.

Material: GSB (Granular Sub-Base)

Density: 1,700 kg/m³

Wastage: 6% included

Coverage Visualizations

Coverage Area

Layer Depth Cross-Section

Practical Coverage at 150.0mm Depth

After 6% wastage and 12% compaction allowance

1 × 25kg bag

0.1 m²

1 × 850kg bulk bag

2.8 m²

1 tonne

3.3 m²

2 tonnes

6.6 m²

Your quantity

100.0 m²

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GSB quantity for finished sub-base

GSB must be ordered as loose material for a specified compacted layer.

The defaults apply 12% compaction and 6% wastage.

Representative density: 1,700 kg/m³.
Use drawing dimensions.
Verify field compaction requirements.

How Is Aggregate Coverage Calculated?

Aggregate coverage is calculated from area, finished layer depth, bulk density, compaction allowance, and wastage. For loose aggregate layers, the calculation is similar to gravel coverage. For roadbase, GSB, WMM, crusher run, and MOT Type 1, the important difference is compaction: the loose delivered quantity must be higher than the finished compacted layer volume.

Use FIND QUANTITY when the area and finished depth are known and you want to calculate tonnes or bags. Use FIND AREA when aggregate quantity and finished depth are known and you want to calculate coverage area. Use FIND DEPTH when quantity and area are known and you want to check the achievable layer thickness.

Step 1 — Convert finished depth to metres

Depth (m) = Depth (mm) / 1,000

Aggregate layer depth must be in metres before volume is calculated. For example, a 150 mm sub-base layer is 150 / 1,000 = 0.15 m deep.

Step 2 — Calculate finished layer volume

Finished Volume (m³) = Area (m²) × Finished Depth (m)

If the area is 100 m² and the required finished depth is 150 mm, the finished volume is 100 × 0.15 = 15.00 m³. This is the compacted or final in-place volume required by the work.

Step 3 — Add compaction allowance for sub-base materials

Loose Volume = Finished Volume × (1 + Compaction / 100)

Compacted materials such as GSB, WMM, crusher run, and MOT Type 1 reduce in thickness and voids during rolling or plate compaction. If the finished volume is 15.00 m³ and compaction allowance is 12%, loose volume = 15.00 × 1.12 = 16.80 m³.

Step 4 — Convert loose volume into mass

Loose Mass (kg) = Loose Volume (m³) × Bulk Density (kg/m³)

Aggregate is usually ordered by tonnes. If loose volume is 16.80 m³ and density is 1,700 kg/m³, loose mass = 16.80 × 1,700 = 28,560 kg, or 28.56 tonnes before wastage.

Step 5 — Add wastage to calculate final order quantity

Order Mass = Loose Mass × (1 + Wastage / 100)

Wastage covers handling loss, trimming, edge loss, small level variation, and material left on the ground during spreading. Apply wastage after compaction allowance so the final quantity represents the amount to order.

Step 6 — Calculate coverage area when quantity is fixed

Coverage Area = Available Mass / [Density × Depth × (1 + Compaction / 100) × (1 + Wastage / 100)]

When quantity is fixed, compaction and wastage reduce the practical coverage area. For example, 20 tonnes of GSB will cover less area at 150 mm finished depth when 12% compaction and 6% wastage are included.

Step 7 — Calculate achievable depth from quantity and area

Finished Depth = Available Mass / [Area × Density × (1 + Compaction / 100) × (1 + Wastage / 100)]

This mode is useful when aggregate is already available on site. The calculator converts quantity into usable finished volume, divides it by the area, and returns the practical layer depth.

Optional — Estimate nominal M20 concrete yield

Concrete Volume = Aggregate Mass / (0.84 × Aggregate Density)

The 0.84 factor is a rough cross-check for the bulk coarse aggregate required per cubic metre of nominal 1:1.5:3 concrete. Use this only for approximate planning, not for structural concrete mix design.

Worked Aggregate Coverage Examples

The examples below show how to enter real site measurements into the aggregate coverage formula. Each example lists the calculator inputs first, then applies the formula step by step: calculate area, convert depth, calculate finished volume, adjust for compaction, convert to tonnes using bulk density, and add wastage where required.

Formula reminder: use FIND QUANTITY when area and depth are known, FIND AREA when quantity and depth are known, and FIND DEPTH when quantity and area are known. For compacted layers, calculate finished volume first, then add compaction allowance before converting to tonnes.

Example 1 — Calculate GSB Required for a Road Sub-Base

Calculator mode: FIND QUANTITY

A road sub-base is 50 m long and 8 m wide. The required finished compacted thickness is 150 mm. The selected material is GSB with an assumed loose bulk density of 1,700 kg/m³. A 12% compaction allowance is added to estimate loose material quantity, and a 6% wastage allowance is added for handling, trimming, edge loss, and level correction.

Input details

These are the values the user would enter or select in the calculator before reading the result.

Input	Value	Why it matters
Calculator mode	FIND QUANTITY	Use this mode when the area and finished layer depth are known, and the calculator must find the aggregate quantity to order.
Material	GSB / granular sub-base	Well-graded sub-base aggregate used below road and pavement layers.
Bulk density	1,700 kg/m³	Used to convert loose aggregate volume into weight.
Length	50 m	Road section length.
Width	8 m	Road section width.
Finished depth	150 mm	Required compacted layer thickness.
Compaction allowance	12%	Allows for loose material reducing during compaction.
Wastage	6%	Allows for handling loss, trimming, edge loss, and site variation.

Step-by-step calculation

The table below shows how the input values are placed into the formula and converted into the final result.

Step	Formula / Substitution	Result
Calculate area	Area = Length × Width	50 × 8 = 400.00 m²
Convert depth to metres	Depth (m) = Depth (mm) / 1,000	150 / 1,000 = 0.15 m
Calculate finished volume	Finished Volume = Area × Finished Depth	400.00 × 0.15 = 60.00 m³
Add compaction allowance	Loose Volume = Finished Volume × 1.12	60.00 × 1.12 = 67.20 m³
Convert loose volume to mass	Loose Mass = Loose Volume × Density	67.20 × 1,700 = 114,240 kg
Convert to tonnes before wastage	Tonnes = kg / 1,000	114,240 / 1,000 = 114.24 tonnes
Add wastage	Order Mass = Loose Mass × 1.06	114,240 × 1.06 = 121,094 kg
Final tonnes	Final Tonnes = 121,094 / 1,000	121.09 tonnes

Result

The 50 m × 8 m GSB road sub-base requires approximately 121.09 tonnes of aggregate, assuming 150 mm finished compacted depth, 12% compaction allowance, and 6% wastage.

Practical note

For road layers, the required depth is usually a compacted finished thickness. Loose delivery quantity must allow for compaction. Always check the project specification, compaction method, layer thickness, and field density requirement before final ordering.

Example 2 — Calculate MOT Type 1 Required for a Car Park Sub-Base

Calculator mode: FIND QUANTITY

A car park area is 600 m². The required finished compacted sub-base thickness is 200 mm. The selected material is MOT Type 1 / crusher run with an assumed loose bulk density of 1,900 kg/m³. A 20% compaction allowance and 5% wastage allowance are used.

Input details

These are the values the user would enter or select in the calculator before reading the result.

Input	Value	Why it matters
Calculator mode	FIND QUANTITY	Use this mode when the car park area and sub-base depth are known.
Material	MOT Type 1 / crusher run	Dense graded sub-base material containing coarse aggregate and fines.
Bulk density	1,900 kg/m³	Used to convert loose volume into weight.
Area	600 m²	Measured car park area.
Finished depth	200 mm	Required compacted sub-base thickness.
Compaction allowance	20%	Allows for loose material reducing during compaction.
Wastage	5%	Allows for trimming, spreading loss, and uneven subgrade correction.

Step-by-step calculation

The table below shows how the input values are placed into the formula and converted into the final result.

Step	Formula / Substitution	Result
Use measured area	Area = 600 m²	600.00 m²
Convert depth to metres	Depth (m) = Depth (mm) / 1,000	200 / 1,000 = 0.20 m
Calculate finished volume	Finished Volume = Area × Finished Depth	600.00 × 0.20 = 120.00 m³
Add compaction allowance	Loose Volume = Finished Volume × 1.20	120.00 × 1.20 = 144.00 m³
Convert loose volume to mass	Loose Mass = Loose Volume × Density	144.00 × 1,900 = 273,600 kg
Convert to tonnes before wastage	Tonnes = kg / 1,000	273,600 / 1,000 = 273.60 tonnes
Add wastage	Order Mass = Loose Mass × 1.05	273,600 × 1.05 = 287,280 kg
Final tonnes	Final Tonnes = 287,280 / 1,000	287.28 tonnes

Result

The car park sub-base requires approximately 287.28 tonnes of MOT Type 1 / crusher run after compaction allowance and wastage.

Practical note

This example shows why compacted sub-base quantities become large quickly. A 200 mm finished layer over 600 m² equals 120 m³ before compaction adjustment. For large paved areas, even a small change in depth has a major effect on tonnage.

Example 3 — Check Coverage Area from a Fixed Aggregate Quantity

Calculator mode: FIND AREA

A contractor has 20 tonnes of GSB available and wants to know how much area it can cover at 150 mm finished compacted depth. The assumed loose bulk density is 1,700 kg/m³. A 12% compaction allowance and 6% wastage allowance are considered.

Input details

These are the values the user would enter or select in the calculator before reading the result.

Input	Value	Why it matters
Calculator mode	FIND AREA	Use this mode when aggregate quantity and finished depth are known, and the calculator must find coverage area.
Material	GSB / granular sub-base	Used for road and pavement sub-base layers.
Bulk density	1,700 kg/m³	Used to convert available mass into loose volume.
Quantity	20 tonnes	Available aggregate quantity.
Finished depth	150 mm	Target compacted layer thickness.
Compaction allowance	12%	Reduces practical coverage because extra loose material is needed for compaction.
Wastage	6%	Reduces practical coverage because some material is lost or used in trimming and level correction.

Step-by-step calculation

The table below shows how the input values are placed into the formula and converted into the final result.

Step	Formula / Substitution	Result
Convert tonnes to kg	Mass = Tonnes × 1,000	20 × 1,000 = 20,000 kg
Calculate loose volume available	Loose Volume = Mass / Density	20,000 / 1,700 = 11.765 m³
Remove wastage allowance	Usable Loose Volume = Loose Volume / 1.06	11.765 / 1.06 = 11.099 m³
Convert loose volume to finished volume	Finished Volume = Usable Loose Volume / 1.12	11.099 / 1.12 = 9.910 m³
Convert depth to metres	Depth (m) = Depth (mm) / 1,000	150 / 1,000 = 0.15 m
Calculate coverage area	Area = Finished Volume / Depth	9.910 / 0.15 = 66.07 m²

Result

20 tonnes of GSB covers approximately 66.1 m² at 150 mm finished compacted depth after allowing for 12% compaction and 6% wastage.

Practical note

When quantity is fixed, compaction and wastage reduce the practical area covered. The calculator should not show the same coverage area before and after these allowances.

Example 4 — Check Finished Depth from a Fixed Quantity and Area

Calculator mode: FIND DEPTH

A user has 30 tonnes of 20 mm crushed aggregate and wants to spread it over a 120 m² area. The aggregate density is assumed to be 1,600 kg/m³. A 5% wastage allowance is applied. No compaction allowance is used because this example is for loose aggregate coverage, not compacted roadbase.

Input details

These are the values the user would enter or select in the calculator before reading the result.

Input	Value	Why it matters
Calculator mode	FIND DEPTH	Use this mode when aggregate quantity and area are known, and the calculator must find achievable depth.
Material	20 mm crushed aggregate	Common coarse aggregate used for construction, drainage, and general filling.
Bulk density	1,600 kg/m³	Used to convert aggregate mass into volume.
Quantity	30 tonnes	Available aggregate quantity.
Area	120 m²	Area to cover.
Wastage	5%	Used to estimate practical depth after spreading and handling loss.
Compaction allowance	0%	No compaction allowance used in this loose aggregate example.

Step-by-step calculation

The table below shows how the input values are placed into the formula and converted into the final result.

Step	Formula / Substitution	Result
Convert tonnes to kg	Mass = Tonnes × 1,000	30 × 1,000 = 30,000 kg
Calculate loose volume	Volume = Mass / Density	30,000 / 1,600 = 18.750 m³
Remove wastage allowance	Usable Volume = Volume / 1.05	18.750 / 1.05 = 17.857 m³
Use measured area	Area = 120 m²	120.00 m²
Calculate depth in metres	Depth = Usable Volume / Area	17.857 / 120 = 0.1488 m
Convert depth to mm	Depth (mm) = Depth (m) × 1,000	0.1488 × 1,000 = 148.8 mm

Result

30 tonnes of 20 mm crushed aggregate spread over 120 m² gives approximately 149 mm practical depth after a 5% wastage allowance.

Practical note

FIND DEPTH is useful when material is already available on site and the user wants to check whether it can achieve the required layer thickness.

Example 5 — Convert Three Brass of 20 mm Aggregate to Tonnes

Calculator mode: FIND QUANTITY

A supplier quotes 3 brass of 20 mm aggregate. The user wants to convert this volume into tonnes. One brass is taken as 100 cubic feet, or approximately 2.832 m³. The assumed aggregate density is 1,600 kg/m³.

Input details

These are the values the user would enter or select in the calculator before reading the result.

Input	Value	Why it matters
Calculator mode	FIND QUANTITY	Use this mode when a volume quantity is known and the calculator must estimate weight.
Material	20 mm aggregate	Common crushed coarse aggregate.
Bulk density	1,600 kg/m³	Used to convert volume into mass.
Quantity	3 brass	Local volume-based aggregate quantity.
Conversion	1 brass = 2.832 m³	Used to convert brass into cubic metres.
Wastage	0%	No wastage added in this simple unit conversion example.

Step-by-step calculation

The table below shows how the input values are placed into the formula and converted into the final result.

Step	Formula / Substitution	Result
Convert brass to cubic metres	Volume = Brass × 2.832	3 × 2.832 = 8.496 m³
Convert volume to mass	Mass = Volume × Density	8.496 × 1,600 = 13,594 kg
Convert kg to tonnes	Tonnes = kg / 1,000	13,594 / 1,000 = 13.59 tonnes

Result

Three brass of 20 mm aggregate is approximately 8.496 m³, which equals about 13.59 tonnes at 1,600 kg/m³.

Practical note

Brass is a volume unit, while many suppliers quote aggregate by tonnes. The conversion depends on density, so 3 brass of lightweight aggregate and 3 brass of dense crushed stone will not weigh exactly the same.

Example 6 — Estimate Nominal M20 Concrete Yield from Aggregate Quantity

Calculator mode: OPTIONAL CONCRETE YIELD CHECK

A user has approximately 13.59 tonnes of 20 mm aggregate and wants to estimate how much nominal M20 concrete it could produce. This is only a cross-check, not a structural mix design. The assumed aggregate density is 1,600 kg/m³ and the calculator uses a 0.84 coarse-aggregate factor for nominal 1:1.5:3 concrete.

Input details

These are the values the user would enter or select in the calculator before reading the result.

Input	Value	Why it matters
Calculator mode	Optional concrete yield	Use this only as a rough cross-check when aggregate is being related to nominal concrete quantity.
Material	20 mm aggregate	Coarse aggregate used in nominal M20 concrete.
Aggregate mass	13,594 kg	Mass calculated from 3 brass in the previous example.
Aggregate density	1,600 kg/m³	Used with the 0.84 factor to estimate concrete yield.
Coarse aggregate factor	0.84	Approximate bulk coarse aggregate per 1 m³ of nominal 1:1.5:3 concrete.

Step-by-step calculation

The table below shows how the input values are placed into the formula and converted into the final result.

Step	Formula / Substitution	Result
Use aggregate mass	Aggregate Mass = 13,594 kg	13,594 kg
Calculate denominator	0.84 × Aggregate Density	0.84 × 1,600 = 1,344 kg per m³
Estimate concrete volume	Concrete Volume = Aggregate Mass / (0.84 × Aggregate Density)	13,594 / 1,344 = 10.11 m³

Result

13.59 tonnes of 20 mm aggregate can produce approximately 10.11 m³ of nominal M20 concrete by this rough cross-check method.

Practical note

This is not a replacement for concrete mix design. For RCC or structural work, use an approved mix design or project specification. Cement, sand, water, moisture correction, grading, and bulking must also be considered.

Calculator Limitations & Assumptions

Concrete coverage uses nominal M20 mix only: the 1:1.5:3 assumption will not match a design mix. Use the Concrete Mix Calculator for design-mix quantities.
Compaction factors are typical ranges: actual values depend on gradation, equipment, moisture, and passes. Follow the project geotechnical specification for structural bases.
Coverage assumes a regular area: divide road curves, irregular embankments, and tapered sections into measured sub-sections.
Density varies by quarry: for orders above about 50 tonnes, use supplier-tested bulk density rather than reference values.
RCA properties are highly variable: recycled material varies by demolition source in density, absorption, and gradation. Use a tested custom value.

Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.

FAQ

12-15% is typical for GSB Grade I/II under IRC SP:49, depending on gradation and compaction equipment. This page defaults to 12% — check your project specification for the exact requirement.

1,700 kg/m³ is a common representative value; actual density varies by source and grading. Use a supplier-tested value for orders above about 50 tonnes.

No — GSB is the lower, coarser sub-base layer; WMM is a denser road-base course placed above it. Calculate each layer separately with its own defaults (see the WMM Coverage Calculator).

At 150mm finished depth, 1 tonne of GSB at 1,700kg/m³ covers approximately 3.92m² before compaction allowance. To achieve 150mm compacted depth, order 12–15% more loose material. With a 12% factor, practical finished coverage is about 3.50m² per ordered tonne. The calculator applies the selected compaction factor automatically.

Because brass is a volume unit and 1 brass equals 2.832m³, coverage at a given depth is the same for every material: 28.3m² at 100mm, 18.9m² at 150mm, and 14.2m² at 200mm. Weight and cost per brass differ according to material density.

For nominal M20 concrete, coarse aggregate is approximately 0.84m³ bulk volume per cubic metre of concrete. For a 10m × 8m × 0.125m slab, concrete volume is 10m³ and coarse aggregate is 10 × 0.84 × 1,600 = 13,440kg or 13.44 tonnes. With 7% site wastage, allow approximately 14.38 tonnes.

Both use the same core coverage formula. Aggregate Coverage adds compaction input for GSB, WMM and MOT Type 1, brass input, optional concrete-volume output, and construction-oriented depth presets from 50–300mm. Gravel Coverage is designed for landscaping and decorative applications with shallower depths and prominent bag-count outputs.

Before loose-material allowance, 1 tonne covers 5.26m² at 100mm, 3.51m² at 150mm, and 2.63m² at 200mm. To achieve finished compacted depths, order approximately 15–20% more loose material. For 100m² at 150mm using a 20% factor: 100 × 0.15 × 1.20 × 1,900 = 34,200kg or 34.2 tonnes.

GSB (Granular Sub-Base) is the lower, coarser layer placed directly on the sub-grade to distribute load and improve drainage. WMM (Wet Mix Macadam) is a denser, well-graded layer placed above GSB as the road base course, mixed with water before compaction. Most road pavements use both layers together, calculated and ordered separately.

Aggregate for sub-base and road-base layers is delivered and spread loose, then compacted to its finished thickness with a roller or plate compactor. The loose (as-delivered) volume must be larger than the finished compacted volume — typically 10-25% larger depending on material — or the order will fall short of the specified layer thickness.

No. Aggregate for concrete is batched by weight per the mix design, not spread and compacted as a loose layer. Set compaction to 0% when using this calculator to check coarse-aggregate quantity for concrete, and use the optional concrete-yield output as a rough cross-check only.

FIND AREA answers 'how far will this much aggregate go?' — enter the quantity you already have (or have been quoted) and the target finished depth, and the calculator returns the coverage area after compaction and wastage are removed. This is useful when material is already ordered or delivered and you need to plan the layout.

FIND DEPTH answers 'how thick a layer can I achieve?' — enter the quantity available and the area to be covered, and the calculator returns the achievable finished depth. This helps confirm whether an existing stockpile or delivery is enough to meet a specified layer thickness before spreading begins.

No. It estimates material quantity from your entered dimensions and density only. Layer thickness, compaction requirements, and material specification for road, driveway, and structural sub-base work should come from an approved pavement design or geotechnical report, not from this calculator.