How to order paint for several rooms without rounding twice

Group surfaces by the exact coating

Write one line for each product, colour and finish. Put matching rooms on the same line only when they will use the same coating. White wall paint, white ceiling paint and white primer can still be three different products, so do not combine their litres into one order.

Record the net area, coats, one-coat coverage and selected allowance for each room. If you need those areas first, use the room measurement guide. Door and window openings should be deducted once, before the net area reaches the paint calculation.

Worked example: two rooms, one wall paint

For this example, room A has 30 m² of measured net wall area and room B has 20 m². Both will use the same paint at an assumed 12 m²/L per coat, two coats and a 5% allowance. These are stated example inputs, not measurements of a completed project or a claim about a named paint.

The same combined result comes from (30 + 20) × 2 ÷ 12 × 1.05 = 8.75 L, because the coverage, coats and allowance match. In the paint calculator, choose area mode and enter 50 m², two coats, Product coverage (optional) of 12 m²/L and 5% allowance. Confirm that Coverage used reads 12 m²/L.

Round the shared order once

Suppose this particular paint is available only in 5 L and 1 L tins. The table compares separate room orders with one combined order using those hypothetical sizes. The calculator may suggest other sizes; confirm which ones the retailer actually sells for your paint.

Pooling the requirement saves 1 L in this example. It does not necessarily produce the lowest price: two 5 L tins may cost less than a 5 L tin and four small tins. Compare the price of the complete order and decide whether extra volume is useful.

The unrounded application estimate is 50 × 2 ÷ 12 = about 8.33 L. The selected allowance adds about 0.42 L, giving 8.75 L. Buying 9 L adds a further 0.25 L through container rounding. If actual consumption matches the estimate, about 0.67 L remains; it includes the allowance and is not guaranteed usable repair stock.

If the rooms need different coverage or coats

Keep separate calculations when the surfaces or coat counts differ, even if the paint is identical. Add the unrounded paint requirements, including each room’s chosen allowance, then choose containers for their sum. Do not average the coverage rates and apply that average to the combined area.

For example, keep room A at 30 m², two coats, 12 m²/L and 5% allowance: 5.25 L. If room B’s 20 m² needs the same two coats but covers only 8 m²/L, its requirement becomes 20 × 2 ÷ 8 × 1.05 = 5.25 L. The shared order requirement is 10.50 L, not the earlier 8.75 L. If room B instead uses another product, colour or finish, keep its purchase separate too.

Use coverage in the unit shown

Product coverage is a rate for one coat. In Metric mode the paint and primer fields use m²/L; in Imperial mode they use sq ft/US gal. Enter the rate from the product label in the matching unit. A US gallon is different from an imperial gallon.

A positive Product coverage (optional) value overrides the selected surface preset. If you want to compare presets again, set the product value to 0. Check Coverage used after making the change. Changing coverage does not remove an extra coat or reverse a previously deducted opening.

Keep a short order record

Plan primer as its own order with the primer calculator. For applicators, protection and other supplies, follow the painting supplies checklist. The allowance and pack rounding guide applies the same distinction to flooring packs.