Primer Calculator
A primer calculator divides wall area by 300 sq ft per gallon: a 40 × 8 ft wall (320 sq ft) needs 1.07 gallons of primer, or $26.67 at $25 a gallon.
How much primer to buy
A primer calculator estimates how many gallons of primer a surface needs before painting. Enter the length and the height of the area in feet. The tool divides the area by 300 square feet per gallon and multiplies by your price per gallon.
Primer works like a first thin coat that seals the surface so the paint over it goes on evenly. You need enough to cover every wall once.
The formula in plain words
- Area = length × height. For a room, use the total wall length for length.
- Gallons = area ÷ 300.
- Cost = gallons × price per gallon.
The tool does not round up to whole cans. It gives an exact fraction and a matching cost, so plan to buy the next full can.
Worked example: a long wall
A wall runs 40 ft and is 8 ft tall. Primer costs $25 per gallon.
- Area: 40 × 8 = 320 sq ft
- Primer: 320 ÷ 300 = 1.07 gallons
- Cost: 1.0667 × $25 = $26.67
A gallon will cover it, but not a quart, and you will have a small amount left.
Worked example: a whole bedroom
A bedroom is 14 ft by 12 ft with 8 ft ceilings.
- Wall length: 14 + 12 + 14 + 12 = 52 ft
- Area: 52 × 8 = 416 sq ft
- Primer: 416 ÷ 300 = 1.39 gallons
- Cost: 1.3867 × $25 = $34.67
Buy 2 gallons if you plan on a second coat or want some for touch-ups. In that case, enter your wall area twice, or double the answer.
Primer needed by wall area
| Wall area | Gallons of primer |
|---|---|
| 200 sq ft | 0.67 |
| 320 sq ft | 1.07 |
| 416 sq ft | 1.39 |
| 600 sq ft | 2.00 |
| 1,000 sq ft | 3.33 |
How to measure walls
Measure each wall’s length along the floor and its height from the floor to the ceiling. Add the lengths to get the perimeter, then multiply by the height.
Doors and windows reduce the area. A standard door is about 20 sq ft and a typical window is 15, but you can skip subtracting them for a small safety margin. Ceilings are measured separately as length times width.
If the room has a lot of trim, primer it too, but count it separately. Trim surfaces are narrow, so they add little area.
When primer matters
New drywall is porous and drinks paint unevenly, so a primer coat evens it out. Stains from water, smoke, or marker bleed through ordinary paint, and a stain-blocking primer is made for them. Bare wood needs a sealing coat to stop knots and tannins from showing. Major color changes go on better over a tinted primer.
For a wall that is clean, sound, and already painted in a similar color, a primer may be unnecessary. Read the label on your paint and the primer for guidance.
Quarts, gallons, and five-gallon pails
Primer comes in more than one container size. A job under about a third of a gallon can use a quart. For an entire house, a five-gallon pail is usually the better price per gallon. Use the cost line here with your own price per gallon, then compare it against the pail price divided by five.
Common mistakes
- Skipping the second coat on stained or patched areas. One thin coat may not block the stain.
- Using the paint’s coverage number. Primer is often rated differently, so go by the primer’s label.
- Priming without cleaning. Dust and grease keep it from sticking.
- Buying too little. Running out mid-wall leaves a visible line where the batch changes.
- Ignoring the ceiling. If you are painting it too, measure it separately.
Related calculators
When the primer is dry, the paint calculator estimates the finish coats. If you are hanging new board, count sheets with the drywall calculator. Before walls are closed, the insulation calculator counts batts for the cavities.
Frequently asked questions
How much primer do I need for a room?
Add up the wall area and divide by 300 sq ft per gallon. A room whose walls total 320 sq ft needs 1.07 gallons for one coat.
How do I find the wall area?
Add the lengths of all the walls to get the room's perimeter, then multiply by the ceiling height. A 14 ft by 12 ft room has a 52 ft perimeter, and at 8 ft high that is 416 sq ft.
Does one gallon of primer always cover 300 sq ft?
No. 300 sq ft is a typical figure. New drywall, bare wood, and rough surfaces soak up more, and some primers list a higher or lower number on the can. Use the label's coverage if it differs.
Do I need primer on already painted walls?
Not always. Primer matters most on new drywall, stains, bare wood, patched spots, and big color changes. Check the paint label, since some paints include primer.
Does the result round up to whole gallons?
No, it shows gallons to two decimals. You buy a whole can, so round up, or buy a quart if the extra is small.
Estimates only. Verify quantities with your supplier before purchasing.