Paint Calculator: Gallons for Walls and Ceilings

How many gallons of paint do you need?

Use this calculator when you know the paintable square footage and need a gallon count. The formula is area times coats, divided by the coverage rate for the selected product. The result shows base gallons and rounds the purchase quantity up.

Sherwin-Williams maintains a paint calculator to estimate how much paint a project needs. Its tool is a useful reminder that quantity starts with measured scope. This BuilderBuzz calculator keeps the coverage input visible so you can replace the 350-square-foot default with the label or product-data figure.

The default is a planning input for a smooth interior surface, not a guaranteed spread rate. Surface porosity, texture, application method, color change, and the selected coating can change the order. Primer, trim paint, caulk, masking, and spray loss remain separate.

How to use the paint calculator

1. Measure the length and height of each wall or ceiling plane and calculate its square footage.
2. Add the areas that receive the same coating.
3. Subtract only openings large enough to change the order materially.
4. Enter the planned number of coats. Two is the editable default, not a rule for every product and surface.
5. Enter the coverage rate from the can label or product data sheet. Use 350 only for an early planning pass.
6. Review both the base result and the rounded whole-gallon purchase quantity.

Split the takeoff when walls, ceilings, or rooms use different products, colors, or coverage rates. Combining them under one average can hide where the extra gallon is actually needed.

The paint quantity formula

The calculator uses:

Base gallons = paintable area × coats ÷ coverage per gallon

Gallons to buy = ceiling(base gallons)

For 800 square feet at two coats and 350 square feet per gallon, the exact result is 4.571 gallons. Rounded to one decimal place for the takeoff, that is 4.6 gallons. The whole-gallon order is 5 gallons. Keep the exact result in the estimate and the rounded quantity in purchasing so the reason for the order remains visible.

Worked example: a 1,050-square-foot repaint

A repaint has 1,050 square feet of paintable wall area. At two coats and 350 square feet per gallon:

1,050 × 2 ÷ 350 = 6 gallons

Because the result is already whole, the purchase quantity stays at six gallons. If the selected product lists a different coverage rate, change that input and recalculate. If primer is required, run it as a separate coating with its own area, coat count, and coverage rate rather than adding it to the finish-paint result.

Coverage and ordering notes

Coverage is product-specific. Sherwin-Williams describes Emerald Interior as offering coverage with a formula that hides dark colors. The manufacturer makes that claim on the Emerald product page. Sherwin-Williams says Duration Home provides outstanding coverage in fewer coats. That separate product description illustrates why one coat count should not be assigned to every paint.

Treat those as product claims, not universal takeoff constants. Use the actual product documentation and substrate condition for the job. Keep product line, sheen, base, color formula, and lot information with the order. Box multiple containers together when color uniformity matters and the manufacturer instructions allow it.

Use the paint sheen guide before finalizing separate wall, ceiling, bath, and trim products. For exterior wood, the exterior paint vs stain guide helps decide which coating system belongs in the takeoff. For a new brick scope, use the brick calculator for unit quantity; this paint tool only estimates coating from paintable face area.

What the gallon result does not include

This calculator does not automatically add primer, caulk, masking material, tray and roller loss, sprayer and hose loss, overspray, touch-up stock, or trim enamel. Keep those visible as separate estimate lines.

Primer needs its own coverage and coat assumptions, especially on new drywall, stains, raw wood, patched areas, or a large color change. Trim also needs a separate measurement method because base, casing, doors, and crown do not behave like broad wall area. For sprayed work, add loss according to the equipment, masking plan, and surface rather than silently reducing the coverage input without a note.

Paint calculator reference

Item	Calculator default or rule	Estimating note
Coverage	350 sq ft/gal	Replace with product and surface data
Coats	2	Change to the specified system
Base gallons	Decimal result	Keep for the takeoff record
Purchase quantity	Round up	Order whole gallons unless packaging differs
Openings	Subtract selectively	Remove only openings that materially change quantity
Primer and trim	Not included	Run as separate products

Paint calculator FAQ

How many square feet does one gallon of paint cover?

This calculator starts at 350 square feet per gallon as an editable planning input. Use the coverage rate for the selected product and surface when available.

Should I subtract windows and doors?

Subtract openings that materially change the order. Keep the rule consistent and record gross area, deductions, and net area separately.

Why does the calculator round up?

The decimal result is useful for the takeoff, but purchasing usually happens in available container sizes. The calculator rounds to whole gallons for its order result.

Does the result include primer?

No. Calculate primer separately with its own paintable area, coat count, and coverage rate.