How it works
The calculator adds up the surfaces you’re priming, subtracts doors and windows from the walls, multiplies by the number of coats and divides by the primer’s coverage.
Wall area = 2 × (Length + Width) × Height
Ceiling area = Length × Width
Doors & windows = Σ (Width × Height × Count)
Area to prime = Wall area − Doors & windows + Ceiling area
Primer = Area to prime × Coats ÷ Coverage × (1 + Extra ÷ 100)
Cans = ⌈ Primer ÷ Can size ⌉
⌈ ⌉ means rounding up to the next whole number: you can’t buy part of a can. The primer volume itself is not rounded. In “known area” mode the known area takes the place of the wall and ceiling areas, and doors and windows are subtracted from it.
For the worked example above – a 14 × 12 ft room with 8 ft walls, primed on the walls and the ceiling – the walls are 2 × (14 + 12) × 8 = 416 sq ft and the ceiling is 14 × 12 = 168 sq ft. One door of 3 ft × 6 ft 8 in (20 sq ft) and two windows of 3 × 4 ft (24 sq ft) come off the walls, leaving 372 sq ft of wall. Together with the ceiling that is 540 sq ft. At Benjamin Moore Fresh Start 046’s 400 sq ft per gallon, one coat takes 540 ÷ 400 = 1.35 gallons, so you’d buy two one-gallon cans – or one gallon and two quarts, if you set the can size to quarts and compare prices.
Primer is chosen for the surface, not the color: stain blocking, adhesion to glossy finishes, sealing new drywall or bare wood. That is why its coverage often differs from the finish paint you’ll put on top, and why the calculator asks for the primer separately. Pick your primer from the list to apply the spread rate printed on its manufacturer’s technical data sheet. Where a sheet gives a range, the calculator uses the low end, so the estimate leans towards more primer rather than less. Zinsser’s Bulls-Eye 1-2-3 sheet lists a practical rate for non-porous and for porous surfaces; the calculator uses the porous figure of 350 sq ft per gallon. If your primer isn’t listed, choose “Your own coverage” and type the figure from the can.
Porosity matters more for primer than for most topcoats, because priming is often the first coat on a raw surface. New drywall, fresh joint compound, skim-coated or bare plaster and unpainted wood soak up more material than a wall that has been painted before. PPG’s data sheets state their coverage for smooth, nonporous surfaces and note that the figures don’t include loss from surface irregularities and porosity. Benjamin Moore’s sheet says coverage depends on surface texture and porosity. If you’re priming a lot of fresh drywall or rough masonry, use the lower rate a data sheet gives or add an extra allowance, and buy the last can once you see how far the first one goes.
Measurements, coverage and can size are converted to one internal unit before calculating, so you can enter the room in feet and inches and pick gallons or quarts for the cans. One US gallon is 231 cubic inches (NIST Handbook 44) and equals four quarts. In metric, coverage is in m² per liter; can sizes vary by brand and market, so none is filled in – enter the size you’ll buy to see the number of cans. The coverage used is shown with the result, and the data sheets behind every product are linked under Sources.
Worked example
Inputs
- Measure by
- Room length × width × height
- What you’re priming
- Walls and ceiling
- Room length
- 14 ft
- Room width
- 12 ft
- Wall height
- 8 ft
- Doors & windows (not primed) 1
- Width 3 ft, Height 6 ft 8 in ft, Count 1
- Doors & windows (not primed) 2
- Width 3 ft, Height 4 ft, Count 2
- Number of coats
- 1
- Primer product
- Benjamin Moore Fresh Start High-Hiding All-Purpose Primer 046 – 400 sq ft/gal (≈ 9.8 m²/L)
- Extra primer
- 0 %
- Can size
- 1 gal
Result
- Wall area
- 416 sq ft
- Ceiling area
- 168 sq ft
- Doors & windows
- 44 sq ft
- Area to prime
- 540 sq ft
- Coverage used
- 400 sq ft/gal
- Primer needed
- 1.35 gal (5.4 qt)
- Cans
- 2
Common mistakes
- Using the coverage of your topcoat for the primer. Primers have their own spread rate, and some list a lower figure for porous surfaces – pick the primer or type the number on its can.
- Forgetting that porous surfaces drink primer. New drywall, fresh joint compound, bare plaster and raw wood can take noticeably more than a data sheet’s smooth-surface figure. Add an extra allowance or use a primer’s porous-surface rate.
- Subtracting doors and windows from a ceiling. Openings come off the walls; if you prime the ceiling only, leave the openings list empty.
- Entering a net area in “known area” mode and then listing the openings again, which subtracts them twice.
- Assuming one coat is always enough. Some data sheets say one or two coats – for example over severe stains or before a big color change. Check the label for your surface.
Assumptions and limits
- In room mode, all four walls are primed to the full wall height you enter, and the ceiling is a rectangle of length × width. Sloped ceilings, stairwells and partial walls need the area entered directly.
- Doors and windows are rectangles of width × height × count, subtracted from the walls in room mode and from the known area in area mode.
- Coverage is the documented spread rate of the primer you pick – the low end of the range on its data sheet, or its porous-surface figure where one is given – or the figure you type. It applies to every coat.
- Manufacturers’ rates assume a surface prepared as their sheet describes. Real coverage depends on the surface’s texture and porosity and on how you apply the primer.
- Cans are rounded up to whole containers of the size you choose; the calculator does not mix gallons and quarts.
Frequently asked questions
How much area does a gallon of primer cover?
It depends on the product and the surface. The data sheets used here list about 400 sq ft per gallon for Sherwin-Williams ProMar 200 Zero VOC and Premium Wall & Wood primers and for PPG Speedhide 6-2 and Seal Grip, 400–450 for Benjamin Moore Fresh Start 046, 400–450 for Zinsser Bulls-Eye 1-2-3 with a practical figure of 350 on porous surfaces, and 8–10 m² per liter for Tikkurila Feelings Universal Primer. The PPG figures are stated for smooth or nonporous surfaces.
Does new drywall need more primer?
New drywall and joint compound are porous, and porous surfaces absorb more primer than a sealed wall. PPG’s data sheets say their coverage figures do not include loss due to surface irregularities and porosity, and Zinsser gives a lower practical rate for porous surfaces (350 vs 450 sq ft per gallon). Use the lower figure or add an extra allowance for fresh drywall.
How many coats of primer do I need?
Usually one, but follow the label. Benjamin Moore’s Fresh Start 046 sheet says to apply one or two coats and that two coats may be required on severe stains. Set the number of coats in the calculator – the primer volume scales with it.
Can I use this for primer on a ceiling only?
Yes. Choose “Ceiling only” in room mode; the ceiling is length × width and the wall height is ignored. Doors and windows are not subtracted from a ceiling.
Is primer included in the paint calculator?
No. Primer and paint have different spread rates, so work them out separately – this calculator for the primer coat, then the paint or ceiling paint calculator for the finish coats.
Sources
- Fresh Start High-Hiding All-Purpose Primer (046) Technical Data Sheet — Benjamin Moore. Primer: "Spread Rate Per Gallon 400 – 450 Sq. Ft." and "Depending on surface texture and porosity." (preset uses 400).
- Seal Grip Interior/Exterior Acrylic Universal Primer/Sealer (17-921 Series) Technical Data — PPG. Primer: "Approximately 400 sq. ft./gal. (37 sq. m/3.78L) per U.S. Gallon (3.78 L) on nonporous surfaces."
- Speedhide Interior Quick-Drying Latex Sealer (6-2) Technical Data — PPG. Primer: "Approximately 400 sq. ft. (37 sq. meters) per U.S. Gallon (3.78L) on smooth, nonporous surfaces." Coverage figures "do not include loss due to surface irregularities and porosity".
- Premium Wall & Wood Interior Latex Primer (B28W08111) Product Data Sheet — Sherwin-Williams. Primer: "Coverage: 400 sq. ft. per gallon @ 4 mils wet;1.5 mils dry".
- ProMar 200 Zero V.O.C. Interior Latex Primer (B28W02600) Product Data Sheet — Sherwin-Williams. Primer: "Coverage: 400 sq.ft.per gallon @ 4.0 mils wet; 1.0 mils dry".
- Feelings Universal Primer Product Data Sheet (06.11.2024) — Tikkurila. Interior wall and ceiling primer: "Theoretical coverage 8-10 m²/l." (preset uses 8).
- Zinsser Bulls-Eye 1-2-3 Water-Based Primer (BEY123-01) Technical Data — Rust-Oleum. Primer: "Approximately 400–450 sq. ft. (37–42m2) per gallon"; practical coverage "450 sq. ft./gal. (11.1 m2/l) non-porous" and "350 sq. ft./gal. (8.6 m2/l) porous" (preset uses 350).
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 3, 2026