Paint Coverage Calculator

Estimate paint needed or area you can cover. Subtract openings, choose coats, set coverage rate, and include waste.

Practical Guide

Paint Coverage Calculator: How Many Gallons (or Liters) You Really Need

Use this reader-first guide with the calculator above to turn room measurements into the right number of paint cans—no guesswork. We’ll show the fastest input method, what actually drives coverage, worked examples in US/metric, and buying tips so your finish looks consistent on the first try.

7–10 min read Updated November 10, 2025

Quick Start

  1. 1 Choose what to solve for: Gallons/Liters needed, or Paintable Area from the cans you have.
  2. 2 Pick a mode: By Dimensions — Walls Only, By Dimensions — Walls + Ceiling, or By Total Area.
  3. 3 Select your coverage rate (ft²/gal or m²/L) from presets or enter a custom label rate. Add coats and a small waste/overlap %.
  4. 4 Enter wall/ceiling sizes and subtract openings (windows/doors) to get net area.
  5. 5 Review the result and round up to whole cans (and typical pack sizes). Note your sheen and color change in the estimate.

Tip: Use the can’s printed coverage as your starting point. Most quality interior wall paints list 350–400 ft² per gallon per coat (≈ 9–11 m²/L).

Watch-out: New drywall, raw wood, masonry, heavy texture, or drastic color changes can slash coverage. In those cases, prime first and lower the coverage rate in the calculator.

Variables & Symbols

  • A Gross area (ft² or m²)
  • Aopen Openings area (doors/windows)
  • Anet Paintable net area = A − Aopen
  • w Overlap/waste %, as a fraction (e.g., 0.10)
  • n Number of coats
  • C Coverage per can (ft²/gal or m²/L)
  • e Application efficiency (0.80–1.00)
  • G Gallons (or L) required
\[ A_{\text{net}} = (A – A_{\text{open}}), \quad A_{\text{res}} = A_{\text{net}} \times (1 + w) \] \[ G = \frac{A_{\text{res}} \times n}{C \times e} \]

Typical defaults: \(e=0.9\) (roller + brush) and \(w=0.05\)–\(0.12\) depending on cutting-in, edges, and texture.

Choosing Your Method

By Dimensions — Walls Only

Fast when you’re repainting and only walls need color.

  • Quick perimeter × height math (or use the calculator’s wall fields).
  • Easy to subtract standard openings.
  • Great for single-room estimates.
  • Misses ceiling unless you add it separately.
  • Irregular nooks need separate entries.
\(A_{\text{walls}} = \text{Perimeter} \times \text{Height} – A_{\text{open}}\)

By Dimensions — Walls + Ceiling

Best for full repaints or new construction.

  • Includes ceilings so you don’t under-order.
  • Works well with “two coats everywhere” assumptions.
  • Ceiling shape/soffits must be measured or estimated.
\(A_{\text{net}} = (\text{Perimeter} \times \text{Height} + A_{\text{ceiling}}) – A_{\text{open}}\)

By Total Area

Use when plans or takeoffs already list paintable area.

  • Fastest workflow—just set coats and coverage.
  • Ideal for multi-room budget pricing.
  • Less visibility into where time/paint goes.
  • Risk of double-counting if openings weren’t removed.
\(G = \dfrac{(A – A_{\text{open}})(1+w)\,n}{C\,e}\)

What Moves the Number

Coverage rating

Printed on the can (e.g., 350–400 ft²/gal). Higher coverage means fewer cans for the same area.

Coats

Two coats roughly double paint volume. Dark-over-light or big color shifts often need a third or a tinted primer.

Surface porosity/texture

New drywall, bare plaster, masonry, texture (orange peel/knockdown) soak up more paint.

Application method

Sprayers can cover fast but overspray reduces efficiency; rollers + back-rolling improve hide.

Edges & cut-ins

Lots of trim, niches, windows, or exposed beams increase overlap and losses.

Conditions

Temperature/humidity affect leveling and recoat times, sometimes increasing needed product.

Worked Examples

Example 1 — US Imperial (Walls Only)

  • Mode: By Dimensions — Walls Only
  • Room: 12 × 15 ft, 9-ft walls → Perimeter = \(2(12+15)=54\) ft
  • Walls area: \(A = 54 \times 9 = 486\) ft²
  • Openings: 1 door (21 ft²) + 2 windows (12 ft² each) → \(A_{\text{open}}=45\) ft²
  • Net area: \(A_{\text{net}} = 486 – 45 = 441\) ft²
  • Coats: \(n=2\)   Waste: \(w=0.10\)   Efficiency: \(e=0.90\)
  • Coverage: \(C = 400\) ft²/gal per coat
1
Adjust for overlap: \(A_{\text{res}} = 441 \times (1+0.10)=485.1\) ft²
2
Total coat-area: \(A_{\text{res}} \times n = 970.2\) ft²
3
Gallons: \(G=\dfrac{970.2}{400 \times 0.90}=2.69 \Rightarrow \mathbf{3~gal}\)
4
If you include the ceiling (12 × 15 = 180 ft²), the same inputs jump to about 4 gal.

Example 2 — Metric (Walls + Ceiling)

  • Mode: By Dimensions — Walls + Ceiling
  • Room: 4.0 × 5.0 m, 2.7-m walls → Perimeter = \(2(4+5)=18\) m
  • Walls area: \(A_{\text{walls}} = 18 \times 2.7 = 48.6\text{ m}^2\)
  • Ceiling: \(A_{\text{ceil}}=4.0 \times 5.0 = 20\text{ m}^2\)
  • Openings: 2 windows (1.2 m² each) + 1 door (1.9 m²) → \(A_{\text{open}}=4.3\text{ m}^2\)
  • Net: \(A_{\text{net}} = 48.6 + 20 – 4.3 = 64.3\text{ m}^2\)
  • Coats: \(n=2\)   Waste: \(w=0.08\)   Efficiency: \(e=0.9\)
  • Coverage: \(C=10\text{ m}^2/\text{L}\) per coat
1
Overlap: \(A_{\text{res}}=64.3 \times 1.08 = 69.44\text{ m}^2\)
2
Coat-area: \(69.44 \times 2 = 138.88\text{ m}^2\)
3
Liters: \(L=\dfrac{138.88}{10 \times 0.9}=15.43 \Rightarrow \mathbf{15\!-\!16~L}\)
4
Buy 1 × 10 L + 1 × 5 L + a small 1 L for touch-ups if available.

Rounding rules: Always round up to whole cans. Many stores accept returns of unopened cans—check policy before you over-order.

Surfaces, Sheens & Variations

Different substrates and finishes shift true coverage. Use the table to adjust your expectations and the calculator’s presets.

ScenarioTypical CoverageCoatsNotes
New drywall (unprimed)200–300 ft²/gal (5–7 m²/L)Primer + 2Primer seals paper/mud so finish coats hide uniformly.
Primed drywall / previously painted (sound)350–400 ft²/gal (9–11 m²/L)2Baseline most calculators use; good roller technique matters.
Textured walls (orange peel/knockdown)250–325 ft²/gal (6–8 m²/L)2Peaks increase surface area; bump waste/overlap.
Dark-to-light or light-to-dark changeVaries2 + tinted primer (often)Tinted primer reduces the third coat risk and saves finish paint.
Trim/doors (semi-gloss)350–450 ft²/gal (9–12 m²/L)2Higher sheen flashes easily—keep a wet edge and back-brush.
Masonry/block (unsealed)125–200 ft²/gal (3–5 m²/L)Block filler + 2Highly porous; specialty primers or fillers required.
  • Confirm your can’s coverage rating against calculator presets.
  • Prime stains, patched zones, or gloss surfaces before finish coats.
  • Record color codes, sheen, and batch numbers for touch-ups.
  • Plan ventilation and recoat windows based on temperature/humidity.
  • Back-roll after spraying for uniform sheen and hide.
  • Buy an extra quart for future dings on high-traffic walls.

Buying, Logistics & Sanity Checks

Selecting Product

  • Sheen: Flat/matte hides imperfections; eggshell/satin cleans better; semi-gloss for trim/doors.
  • Primer: Use for new drywall, stains, drastic color changes, or glossy surfaces.
  • Low-VOC: Improves indoor air quality and odor control during projects.

Application & Workflow

  • Cut-in first, then roll “W” patterns and back-roll to level.
  • Maintain a wet edge; don’t overwork partially set paint.
  • Respect recoat times; forced early recoats reduce hide and can lift.

Sanity Checks

  • Does \(G\) align with can coverage × coats? If not, revisit waste/efficiency.
  • Count whole cans only; plan sizes (1-gal, 5-gal, 1-L, 10-L).
  • Test a 3×3 ft patch: if hide is weak, add a primer step or a third coat to the estimate.

Always follow local codes and manufacturer instructions for prep, application, and safety.

Frequently Asked Questions

How much does 1 gallon of paint cover?
Most interior wall paints list 350–400 ft²/gal per coat on the label (≈ 9–11 m²/L). Porous/rough surfaces and heavy textures reduce this significantly.
Do two coats double the paint I need?
Nearly. Start with the single-coat requirement from your coverage rate, then multiply by the number of coats, and add overlap/waste.
When do I need primer?
For new drywall, stains, glossy surfaces, or a big color change. Tinted primer often saves a third finish coat.
Is spraying or rolling more efficient?
Spraying is fast but can lose product to overspray. Rolling (or back-rolling after spraying) improves transfer and hide. Use the calculator’s efficiency factor to reflect your method.
How do I estimate ceilings?
Include the ceiling area directly (L × W) in Walls + Ceiling mode. Ceilings often need flatter sheens for glare control.
How many cans should I buy?
Round up the calculator’s gallons (or liters) to whole cans and choose pack sizes that minimize overage. Keep one unopened can for touch-ups if returns are allowed.
Do darker colors really need more paint?
Often yes—deep bases can have less hiding power. A matching tinted primer under dark colors reduces total finish paint used.

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