TryBuildCalcWindow Box Soil Calculator (Straight and Tapered Troughs)
Find how much potting mix a window box or trough needs.
🕒 Last updated: September 24, 2026
Inputs
Containers
Applies to every container dimension and to the headspace — switching converts the figures already entered rather than reinterpreting them. Always measure the inside of a container — a thick-walled pot or planter can lose an inch or more of width to its own walls.
Container List
Add a row per size or shape — pots, window boxes and baskets can all go in one job.
Container 1
Window boxes are shallow and dry out fast, so leave the headspace but do not skimp on depth of mix — many are only 6-8 in deep inside.
Fill Level
The more accurate option for a measured container: the mix volume is recomputed at the true fill line, so a tapered pot correctly loses its widest slice.
ℹ️The gap left between the mix and the rim so water soaks in instead of running over the side. Roughly 1 in (2-3 cm) suits a typical pot; allow more on a large planter and less on a shallow window box.
ℹ️Extra mix to cover settling after the first watering, so you are not left topping up from an empty bag. Around 10% is a reasonable allowance; enter 0 to order the bare calculated volume.
Bags & Cost Estimation
ℹ️US bags are labelled in DRY quarts (25.7 to a cubic foot, not the 29.9 a liquid-quart conversion would give); UK, EU and Australian bags are labelled in litres. Both are listed here, so pick whichever matches the bag in front of you.
Potting Mix Needed
2.29 cu ft
58.9 dry qt / 64.9 L / 0.065 m³
Bags to Buy
3
25 dry qt (≈27.5 L / 0.97 cu ft) — large potting mix bag
3 × Window box / trough
Filled to 0.75" (1.9 cm) below the rim — about 89.3% of total container volume.
| Container | Qty | Holds (full) | Mix each | Mix total |
|---|---|---|---|---|
| Container 1 Window box / trough | 3 | 20 dry qt (22 L) | 17.9 dry qt (89.3% full) | 2.08 cu ft (53.6 dry qt) |
| Total (before settling) | 3 | 60 dry qt | — | 2.08 cu ft |
Volume
Total container volume (brim-full): 2.33 cu ft (60 dry qt / 66.1 L)
Mix volume at your fill level: 2.08 cu ft (53.6 dry qt / 59 L)
Order volume (+10% settling): 2.29 cu ft (58.9 dry qt / 64.9 L)
Dry quarts, not liquid quarts
US potting mix bags are labelled in dry quarts — 25.7 to a cubic foot. Your 2.29 cu ft is 58.9 dry qt. A calculator that converted with liquid quarts would tell you 68.6 qt — about 16% more mix than you need.
Assumptions Used
Each container's volume comes from the exact solid its shape describes — a cone frustum for a tapered round pot, a prismatoid for a tapered rectangular planter, a cylinder for a straight-sided pot or filled grow bag, a spherical cap for a rounded basket — computed from the inside dimensions you entered. With the headspace method the mix volume is recalculated at the true fill line, including the fill line's own width, rather than scaling the brim-full volume by a flat percentage — on a tapered container the slice left empty is its widest, so a flat percentage overstates the mix needed. Volumes are held in cubic inches throughout and converted only for display, with US quart figures in dry quarts (67.2 cu in each) to match bag labelling. A 10% settling and top-up allowance is added before the bag count, which is then rounded up to the next whole bag. Per-row volumes shown in the breakdown use largest-remainder rounding so they sum exactly to the printed total. This calculator estimates material quantity only — it does not assess mix recipe, drainage, fertiliser content, or whether a container suits a particular plant.
Container Fill Diagram
Container drawn to the proportions of the inside dimensions entered; mix shown at the computed fill line. Bag shading indicates how much of the last bag the order uses.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Potting Soil & Container Volume Calculator.
Shallow boxes need a smaller headspace than a deep pot
A 24 x 8 in window box 7 in deep inside holds 1,344 cubic inches brim-full — 20 dry quarts, or 22 litres. A 36 in box of the same section holds 2,016 cubic inches, 30 dry quarts. Because they are shallow, the headspace matters proportionally more than on a pot: a full inch is nearly a seventh of a 7 in box, against under a tenth of a 12 in pot. Around three quarters of an inch is usually a better balance on a trough, and this calculator reports each container's own resulting fill percentage so you can see the effect.
Many moulded troughs also taper toward the base, which on something this shallow is a meaningful share of the volume. Switching the container type to the tapered trough option models it as a prismatoid using inside measurements at both the rim and the base, and handles the common case where the length and the width taper by different amounts.
Potting Soil Formula & Method
Every figure is computed in cubic inches and converted only for display. Cubic inches is the natural working unit here because it is the unit the nursery container standard publishes its class volumes in, and because it converts cleanly to both dry quarts and litres.
Step 1 — Container volume, by shape
Round, straight-sided (cylinder):
V = pi x D² x h / 4
Round, tapered (cone frustum):
V = (pi x h / 12) x (D² + D x d + d²)
Rectangular, straight-sided (box):
V = L x W x h
Rectangular, tapered (prismatoid):
V = (h / 6) x [L x W + (L + l) x (W + w) + l x w]
Bowl / rounded basket (spherical cap):
V = (pi x h / 6) x (3 x (D/2)² + h²)
D, L, W = inside dimensions at the RIM
d, l, w = inside dimensions at the BASE
h = inside depth
All five are exact formulas for their solid, not approximations. The frustum reduces to the cylinder when there is no taper, and the prismatoid reduces to the box — so entering equal rim and base dimensions gives the straight-sided answer, as it should. The prismatoid form is used rather than a pyramid frustum because a planter can taper by different amounts along its length and its width, and the prismatoid rule is exact for that case too.
Step 2 — Volume of mix at the fill line
Headspace method:
filled depth h_f = h - headspace
width at fill line = base + (rim - base) x (h_f / h)
V_mix = the SAME shape formula, recomputed with
h_f and the fill-line width
Percentage method:
V_mix = V_container x fill% / 100
(the fill DEPTH is then solved back from V_mix)
This is the step most calculators skip. Scaling the brim-full volume by the depth ratio would treat the empty slice as if it were an average slice, but on a container that is wider at the rim the empty slice is the widest one. For a 12 in rim, 9 in base, 10.5 in deep pot, filling to 9.5 in gives 805 cu in, not the 828 cu in a flat depth-ratio scaling suggests — a 2.9% difference on one pot, and the same 2.9% across every pot in the order. Note also that the two methods are not interchangeable: 90% of that pot's volume sits at 9.67 in of depth, not 9.45 in.
Step 3 — Whole job, plus settling
V_row = V_mix x number of containers in the row
V_total = sum of every row
V_order = V_total x (1 + settling% / 100)
The settling allowance covers the drop in level when a fresh, air-filled mix is watered in for the first time, so you are not left topping up from an empty bag. It is a practical top-up figure rather than a cited standard — 10% is a reasonable default, and 0 gives the bare calculated volume.
Step 4 — Unit conversion, in DRY quarts
1 US dry quart = 67.200625 cu in -> 25.71 per cu ft
1 US liquid quart = 57.75 cu in -> 29.92 per cu ft
1 litre = 61.024 cu in
1 cubic foot = 1,728 cu in
1 US gallon = 231 cu in
dry qt needed = V_order / 67.200625
Bagged potting mix, garden soil and compost sold at US retailers is labelled in US dry quarts. Using liquid quarts — the conversion a general-purpose unit converter will give you — overstates the number of labelled quarts by 16.4%. The calculator reports both figures side by side so the gap is visible rather than assumed.
Step 5 — Bags and cost
bags = ROUNDUP( V_order / bag volume ) (minimum 1)
cost = bags x price per bag
The bag count rounds up because partial bags cannot be bought, and it is always computed from the unrounded order volume — rounding the volume for display first and then dividing can silently drop a bag when the true figure sits just over a whole-bag boundary. Cost follows the whole-bag count, not the volume, because that is what you actually pay.
Containers entered by trade class
V_container = ANSI Z60.2-2025 Table 1 class volume
basis = class minimum / midpoint / maximum
"true to size" reference (standard §1.1.3.4):
cu in = gallons x 231
A class is a permitted range, not a size, so there is no single correct figure to use — the basis is yours to choose. Because such a row has a published volume but no published dimensions, a headspace in inches cannot be applied to it, and the percentage fill level is used for those rows instead. The calculator states this on the row and in the results rather than mixing two fill bases silently.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Container 1 | 3 x Window box / trough — length 24, width 8, depth 7 in |
| Fill Level | 0.75" (1.9 cm) headspace below the rim |
| Settling & Top-Up Allowance | 10% |
| Bag Size | 25 dry qt (≈27.5 L / 0.97 cu ft) — large potting mix bag |
Step 1 — Container Volume, by Shape
| Calculation | Result |
|---|---|
| 24 x 8 x 7 | 1,344 cu in each (20 dry qt) — rectangular box |
| Total brim-full volume across 3 containers | 4,032 cu in (2.33 cu ft) |
Step 2 — Volume of Mix at the Fill Line
| Calculation | Result |
|---|---|
| filled depth = 7 - 0.75 = 6.25 in, straight sides so the volume scales with depth | 1,200 cu in of mix each — 89.3% of the container |
Step 3 — Whole Job, Plus Settling
| Calculation | Result |
|---|---|
| 1,200 cu in x 3 containers | 3,600 cu in (2.08 cu ft) |
| Total mix before settling | 3,600 cu in (2.08 cu ft) |
| 3,600 x (1 + 10 / 100) | 3,960 cu in to order (2.29 cu ft) |
Step 4 — Unit Conversion, in DRY Quarts
| Calculation | Result |
|---|---|
| 3,960 / 67.200625 cu in per dry quart | 58.9 dry qt — the unit US bag labels use |
| 3,960 / 1,728 cu in per cubic foot, and / 61.024 per litre | 2.29 cu ft (64.9 L) |
| The same volume converted with LIQUID quarts (57.75 cu in) instead | 68.6 qt — about 16% more mix than the containers hold |
Step 5 — Bags and Cost
| Calculation | Result |
|---|---|
| One 25 dry qt bag | 1,680 cu in (27.5 L) |
| ROUNDUP(3,960 / 1,680) | 3 bags to buy |
Figures above are rounded for display; the real quantities are always calculated from full, unrounded values.
Therefore, 3 containers need 2.29 cu ft of potting mix — 58.9 dry quarts (64.9 litres) including a 10% settling allowance — which comes to 3 bags of 25 dry qt.
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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.