Understanding your result
The headline is the block filler for the coats, with the method’s loss, and the sub-line is the cans. The three figures show the area, the coverage and the cans.
The table shows the wall area, the openings, the area to fill, the coverage for the block texture, the application loss for rolling or spraying, and then the standard coverage, coats, per-coat, allowance, total and can lines. At this coverage even a single-car garage’s walls take a pail.
Block filler is the first coat of a system. The topcoat is in the stucco and masonry paint calculator, for the same area; the masonry sealer calculator is the clear alternative where the block will not be painted; and the exterior house calculator covers a block house with gables and openings.
How we calculate this
The coverages are the figures on block filler data sheets: about 75 to 125 sq ft per gallon on smooth block and 50 to 80 on split-face and other rough block. The calculator uses 100 and 70. They are low because block filler is a high-solids product applied thick, which the film thickness guide explains with the formula behind every coverage figure. The coverage rates table lists block fillers with the masonry paints.
The siding guide covers block among the wall surfaces, and the primer guide explains why a filler is a primer with a special job.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Smooth block | 100 sq ft per gallon | Block filler data sheets (Behr, Sherwin-Williams Loxon, PPG, Kilz): 75–125 |
| Split-face or rough block | 70 sq ft per gallon | The same sheets: 50–80 |
| Coats | 1 | The usual instruction; 2 for very open block |
| Loss, roller and brush | 5% | Site figure |
| Loss, sprayer | 30% | Overspray of a heavy material, back-rolled |
| Cure before filling | about 28 days for new block and mortar | Block filler instructions |
Assumptions last reviewed October 8, 2026.
The calculator does not model mortar repair, efflorescence removal, waterproofing, or the topcoat. It assumes the block is dry and clean.
Two worked examples
A garage’s block walls, rolled
80 ft of 8 ft smooth block wall, no openings counted, one coat, roller and brush.
- Area: 80 × 8 = 640 sq ft
- Per coat: 640 ÷ 100 = 6.4 gallons
- Filler: 6.4 × 1.05 = 6.72 gallons, buy a 5-gallon pail, a gallon and three quarts
Two pails is the simpler buy. In metric, 24 × 2.4 m is 57.6 m² and 24.6 litres.
Split-face block, sprayed and back-rolled
The same 640 sq ft in split-face block, one coat, sprayed.
- Per coat: 640 ÷ 70 = 9.14 gallons
- Filler: 9.14 × 1.30 = 11.89 gallons, buy two pails and two gallons
Nearly twice the smooth-block figure: the split face is a much larger surface, and the sprayer loses a third.
Where to find your inputs
Wall size. Total length of block wall times its height, minus doors and windows.
Block texture. Smooth standard block, or split-face with a rough, quarried look.
Coats and method. One coat; roller for small walls, sprayer with back-rolling for large or rough ones.
Coverage. On the pail; enter it if it differs from the typical figure.
Common mistakes
- Painting bare block without filler. Pinholes let water in and the paint blisters.
- Treating filler as waterproofing. It is not.
- A thin nap roller. Use 3/4 in or thicker and work it in.
- Filling damp block. Dry block, dry weather.
- Spraying without back-rolling. The filler bridges the pores instead of filling them.
- Skipping the joints. Brush the mortar joints; they are the most porous part.
Questions people ask
- How much block filler do I need?
- About 100 sq ft per gallon on smooth block, 70 on split-face. An 80 ft run of 8 ft block wall is 640 sq ft, which takes 6.4 gallons for one coat, 6.72 with a 5% roller allowance, so a 5-gallon pail and two gallons.
- Do I need block filler before painting block?
- On bare, porous block, yes, for a smooth, pinhole-free finish that lasts. Paint alone sinks into the pores and leaves pinholes that let water in. On block that is already painted and sound, no.
- Is block filler waterproof?
- No. It fills the surface for the paint. Basement walls with water pressure behind them need a masonry waterproofing coating; block filler under ordinary paint will not stop water.
- How many coats of block filler?
- One is normal, worked into the surface with a thick-nap roller and a brush in the joints. A second coat is for very open block or where the first coat still shows pinholes when dry.
- Can I spray block filler?
- Yes, with a pump and tip rated for heavy material, and back-roll it to push it into the pores. Spraying loses about 30% to overspray, which the calculator adds.
- What goes on top of block filler?
- Two coats of exterior acrylic or masonry paint outside, or a good interior paint inside, once the filler has dried for the time on the can.