Coverage Figures: Estimating How Much Product You Need

Every coating carries a coverage figure, usually expressed as square feet per gallon or per litre. It is genuinely useful and it is also the most commonly misread number in the trade, because it describes a smooth, sound, sealed surface applied at a specified film thickness by someone who is not in a hurry. Almost no real job matches that description.

Estimating well is not complicated, but it does require doing three things in order: measure the area, adjust for the surface, and then adjust for the number of coats.

Measure the area properly

Multiply width by height for each wall and add them together. Do not subtract doors and standard windows — the offcuts, the cutting-in and the touching-up will absorb that allowance and more. Do subtract genuinely large openings such as a full glass wall or a wide archway.

For ceilings, length times width. For a deck, the plan area plus roughly a quarter for the edges of the boards if you are coating the sides as well. Awkward shapes are best broken into rectangles rather than approximated in one go.

Adjust for what the surface does to the product

This is the step people skip. A published figure assumes a sealed, smooth surface. Real surfaces vary enormously:

• Sealed, smooth, previously painted: use the published figure.
• New plaster or bare drywall, primed: allow around ten to fifteen per cent more.
• Bare, unprimed porous surfaces: the first coat can use half as much again.
• Light texture or orange peel: add roughly fifteen per cent.
• Heavy texture, render or block: add thirty to fifty per cent — the actual surface area is far larger than the measured area.
• Rough sawn timber and decking: often double the smooth-timber figure on the first coat.

These are working allowances rather than precise science, but they are far closer to reality than the tin’s figure taken literally.

Count coats honestly

Very few products genuinely cover in one coat over anything other than a similar colour. Assume two finish coats unless you are recoating an identical shade, and assume the first coat over primer uses more than the second.

Deep and saturated colours — strong reds, deep blues, anything with a lot of pigment and little white — routinely need three. A tinted primer reduces that, sometimes by a full coat, and is usually cheaper than an extra tin of finish.

The worked example

A room twelve feet by fourteen, walls nine feet high. Perimeter is fifty-two feet; times nine gives four hundred and sixty-eight square feet. The walls are new drywall, primed, so add twelve per cent: about five hundred and twenty-five. Two finish coats: one thousand and fifty square feet of coverage required. At a published three hundred and fifty square feet per gallon, that is three gallons.

Buy three. Buying two and a quart, and discovering on the second coat that you are short, means either a trip out mid-job or a visible batch difference.

Batch numbers and mixing

Where a job needs more than one container of a tinted product, box them — pour all the containers into one large bucket and mix — before starting. Tinting varies slightly between containers, and the difference is invisible in the tin and obvious across a wall.

If boxing is impractical, at least start the second container at a corner rather than in the middle of a wall.

Where coverage figures are missing

Some specialty products, particularly imported decorative finishes, publish coverage only per pass and only as a range, because the figure genuinely depends on how the material is worked. Where that is the case we reproduce the manufacturer’s range rather than picking a number from the middle of it, and we say the figure is per pass rather than per coat.

If a product page anywhere quotes a single confident number for a hand-troweled finish, treat it with suspicion.

A note on waste

Order slightly over, and keep the leftover sealed and labelled with the room and the date. Touch-up material that matches exactly is worth more than the few dollars the extra quart cost, and once a tinted batch is gone it cannot be reproduced perfectly.

Coverage figures for everything we stock are on the product pages in primers and sealers, specialty wall finishes and deck and floor coatings.

Estimating for a second colour or a feature wall

Feature walls are where estimates go wrong most often, because people measure the feature wall and forget that the finish coat still has to be cut in along three edges of it. Cutting in uses far more product per linear foot than rolling does, and on a single wall the cutting-in can be a third of the total.

Where two colours meet, allow an extra quarter of a gallon over the calculation for the smaller area. It sounds generous and it is roughly what a careful cut-in with a brush actually consumes across a room.

Ordering for a phased job

If a project runs over several weekends, buy all of the tinted product at the start even though it will sit for a month. Tint batches drift, and returning to a supplier six weeks later for one more gallon of the same colour frequently produces a slightly different result that shows on a wall lit from one side.

Stored properly — sealed, upright, somewhere frost-free and out of direct sun — water-based coatings keep for a long time. Stir thoroughly before reopening and strain through a paint filter if a skin has formed rather than trying to break it up into the body of the product.