How to calculate paint quantity and compare can sizes

Paint planning has two separate questions: how much liquid the surfaces may need, and how many cans you must buy. The can size and price can change the purchase even when the room stays the same. Start with the real surfaces and the coverage information for your chosen paint.

Use the free paint calculator

Measure actual paintable surfaces

For a separate rectangular room, wall area is 2 × (length + width) × height. Subtract the area of openings you are excluding. Add length × width for the ceiling only if it will receive the same paint and coat count. OKRoomTools can add up to 20 rooms that share those settings.

A 16 × 12 ft room with 8 ft walls has 448 ft² of wall surface. If doors and windows total 42 ft², the net wall area is 406 ft². The ceiling is excluded from this example.

Use known area for open-plan spaces or a single wall

An L-shaped space should not be entered as two imaginary enclosed rooms: their shared boundary would add walls that do not exist. Measure each real wall face instead, subtract the openings, and choose Known paintable area. That mode uses the area directly; it does not subtract openings again or add a ceiling.

It is also useful for one accent wall or a surface measured by another method. Keep different colors, products, ceiling paints, and primer separate. All surfaces in one OKRoomTools paint calculation share one product, color, coverage, and coat count.

Apply coats, coverage, and your allowance

The formula is paintable area × coats ÷ coverage per coat × (1 + allowance percentage / 100). For 406 ft², two coats, illustrative coverage of 400 ft² per US gallon, and 5% extra, the result is 2.1315 US gallons.

The 400 ft² coverage and 5% allowance are example inputs, not product specifications or instructions. Texture, porosity, color changes, preparation, tools, and application can change consumption. Check the label or technical information for the actual paint. BEHR’s estimating guidance and the Lowe’s paint calculator explain why product and surface information matter.

Compare whole-can purchases

If option A is one US gallon at $42, 2.1315 gallons requires three cans. The paint costs $126; $12 for accessories makes a $138 purchase.

If option B is five US gallons at $165, one can is enough. With the same accessories, the purchase is $177. Although its price per gallon is lower, that larger can costs $39 more for this example. OKRoomTools compares each can size independently; it does not find an optimized mix of large and small cans.

Keep units and project costs clear

A US gallon and an imperial gallon hold different amounts. Choose the unit on your product information; OKRoomTools converts the displayed volume and coverage when you switch units. Changing the currency label does not convert prices.

Carry your selected purchase into the project estimate with labor and other expenses. Primer or another paint can be entered there as a separate cost, but that does not calculate its quantity. Planning flooring too? Use the flooring box calculator. These free tools process project inputs locally without an account. Check the complete method and limitations, or return to all planning guides.