TryBuildCalcPaint Calculator with Doors and Windows Room paint estimator with opening deduction
Calculate paint for a whole room, with doors and windows deducted.
🕒 Last updated: July 14, 2026
Inputs
Section 1: Room & Surface
ℹ️Use room dimensions if you'd rather let the calculator work out the area, or enter a known wall area directly.
ℹ️Inside wall-to-wall length of the room.
ℹ️Measured from finished floor to finished ceiling.
ℹ️Off by default for a quick estimate — switch on to deduct doors, windows, or other openings from the gross wall area.
Doors, Windows & Openings
Opening 1
Opening 2
ℹ️Adjusts the suggested coverage rate by up to ±30% for how much paint the surface absorbs.
ℹ️Auto-suggested from Paint Type and Surface Condition — still editable.
ℹ️Drives the suggested wastage allowance below.
ℹ️Auto-suggested from Application Method and Surface Condition — still editable.
Section 2: Can Size & Cost
ℹ️Check your product's label — common sizes are 1, 4, 10, and 20 liters.
Common can sizes
Recommended paint: 6.25 L (1.65 gal) (1 can)
Net Wall Area: 35.73 m²
Room Paint Summary
Gross Wall Area: 40.50 m²
Openings Deducted (3): -4.77 m²
Net Wall Area: 35.73 m²
Wall Paint (before wastage): 5.96 L (1.57 gal)
+ Wastage (5%): 0.29 L (0.08 gal)
Wall Paint Required: 6.25 L (1.65 gal)
Cans & Purchase
Recommended Paint: 6.25 L (1.65 gal) total
Wall Cans: 1 can
Total Cans: 1 can
Quick Insight
Buy at least 6.25 L (1.65 gal) (1 can) from the same production batch to avoid colour or shade mismatch.
Calculated from room dimensions, openings, coverage, and number of coats.
Assumptions Used
Net wall area from gross wall area minus openings, coverage-adjusted for paint type and surface condition | Cans rounded up to the next whole can | Openings (4.77 m²) deducted from the gross area.
Bill of Materials
Main material: 1 cans + recommended tools
+−
Bill of Materials
Main material: 1 cans + recommended tools
Computed items reflect your entered room dimensions and paint details; consumables below are general recommendations — actual needs vary by product and site.
For Your Job
Wall Paint
1 cans
6.25 L total
General Tools & Consumables
Paint roller & tray
Paintbrush (cutting-in)
Masking tape
Drop cloths / floor sheeting
Sandpaper (fine grit)
Approximate results for planning only. Verify with a professional.
Room Paint Visualization (Top View)
Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Room Paint Calculator.
Paint estimate with doors and windows deducted
This page pre-fills a 4 m x 3.5 m room's default 40.5 m² gross wall area with one standard 0.9 m x 2.1 m door (1.89 m²) and two 1.2 m x 1.2 m windows (2.88 m² combined) already deducted — 4.77 m² of openings in total, bringing net paintable area to 35.73 m², which needs 5.96 liters raw for 2 coats or 6.25 liters after 5% wastage, still 1 can of 10 L.
Add Deductions switches on automatically here because this page pre-fills real opening rows — that's the single biggest difference between an area-only estimate and an accurate one, since a plain length x height calculator has no way to remove the space these openings actually take up.
- Gross: 40.5 m². Openings: 4.77 m².
- Net: 35.73 m².
- Cans: 1 (10 L).
How Is Room Paint Calculated?
The calculation starts from the room's gross wall area (from dimensions or a known area), deducts every opening you've added, then works out wall paint, optional ceiling paint, optional primer, and cost step by step.
Step 1 — Gross Wall Area
By dimensions: Gross Wall Area = 2 x (Room Length + Room Width) x Wall Height
By known area: Gross Wall Area = entered directly
Calculation Mode lets you enter room length/width/height or a known wall area directly — both feed the exact same math afterward.
Step 2 — Deduct Openings
Opening Area (per row) = Width x Height x Quantity
Net Wall Area = Gross Wall Area − Sum of every opening row
Each door, window, or other opening is its own row with its own width, height, and quantity — all rows are summed and subtracted together.
Step 3 — Wall Paint
Raw Wall Paint = (Net Wall Area / Coverage) x Coats
Final Wall Paint = Raw Wall Paint x (1 + Wastage / 100)
Coverage is auto-suggested from Paint Type and Surface Condition, and coats are applied to the net (opening-deducted) area.
Step 4 — Primer (Optional Add-On)
Raw Primer = Net Wall Area / Primer Coverage
Final Primer = Raw Primer x (1 + Wastage / 100)
Calculated separately from the wall topcoat and added alongside it — not chosen instead of it — since primer and topcoat are bought in their own cans.
Step 5 — Ceiling (Optional)
By dimensions: Ceiling Area = Room Length x Room Width
By known area: Ceiling Area = entered directly
Final Ceiling Paint = (Ceiling Area / Ceiling Coverage) x Ceiling Coats x (1 + Wastage / 100)
Ceiling paint is tracked as its own stream with its own coverage rate, coat count, and price, since it's typically a different flat/matte product from wall emulsion.
Step 6 — Convert to Cans and Cost
Cans (per stream) = ROUND UP(Final Liters / Can Size)
Cost (per stream) = Final Liters x Price per Liter
Wall, ceiling, and primer cans are rounded up separately, since they're different products. In Multiple Rooms mode, each room's cans round up separately too, unless Same Paint Product for All Rooms is switched on.
Real-World Room Paint Calculation Example
This example uses the active inputs above and follows the same steps from the formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Room size | 4 m x 3.5 m room, 2.7 m height | Determines whether gross wall area comes from dimensions or a direct entry |
| Openings | 3 openings | Deducted from gross wall area before coverage is applied |
| Paint Type / Surface Condition | Interior Emulsion (~12 m²/l) / Standard new putty/plaster finish | Sets the auto-suggested coverage rate below |
| Coverage x Coats | 12.00 m²/liter x 2 | Converts net wall area into raw wall paint liters |
| Wastage | 5% | Adds allowance before rounding to cans |
| Primer | Off | Adds a separate primer line alongside the wall topcoat |
| Ceiling | Not included | Adds ceiling liters and cans as their own stream |
| Can size / Cost estimation | 10.00 L / Off | Rounds final liters to whole cans, and prices them when enabled |
Step 1 — Gross and Net Wall Area
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Gross wall area | 2 x (4 + 3.5) m x 2.7 m | 40.50 m² |
| Net wall area | 40.50 − 4.77 (openings) | 35.73 m² |
Step 2 — Wall Paint
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Raw wall paint | (35.73 ÷ 12.00) x 2 coats | 5.96 liters |
| Final wall paint (incl. wastage) | 5.96 x (1 + 5%) | 6.25 liters |
Step 2 — Cans
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Wall cans | ROUND UP(6.25 ÷ 10.00 L) | 1 can |
| Total cans | Wall cans | 1 can |
Therefore, for a 35.73 m² room, you need 6.25 liters of paint, or 1 can.
Actual paint consumption may vary depending on surface condition, paint quality, application method, and wall preparation. For a whole house, switch on Multiple Rooms and add each room as its own row.
This page keeps things focused on the calculation above. For the full construction guide — verification checklist, reference tables, usage steps, tips, common mistakes, and limitations.See the complete Room Paint Calculator.
Related Calculators
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.