Understanding your result
The headline is the sealer for the number of coats, with the allowance, and the sub-line is the cans. The three figures show the area, the first coat and the cans.
The table lists the area, the sealer type with its coats, the porosity with the first-coat coverage, the first coat, the later coats at their higher coverage, the total with the allowance and the cans. The arithmetic panel shows the first and later coats separately, which is how the products are actually used.
Sealer is the clear alternative to paint on masonry. The stucco and masonry paint calculator is the one for coloured coatings; the concrete floor calculator covers floor paints and sealers on slabs; and the block filler calculator is the first step on bare block that will be painted.
How we calculate this
The first-coat coverages are the typical figures on sealer data sheets by porosity: for penetrating water repellents about 200 sq ft per gallon on dense surfaces, 150 on medium and 100 on porous ones; for film-forming acrylics about 300, 250 and 175. The factor of 1.5 for later coats reflects the lower absorption once the pores are filled. The 10% allowance covers the overspray of a pump sprayer and the roller. The coverage rates table lists sealers with the paints, and the coverage guide explains porosity.
The siding guide covers brick and block as wall surfaces, and the drying guide covers the dry weather sealers need on both sides of the application.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Penetrating, first coat | 200 / 150 / 100 sq ft per gallon (dense / medium / porous) | Silane and siloxane sealer data sheets (Thompson’s, Seal-Krete, Foundation Armor, Rust-Oleum): 100–200 |
| Acrylic, first coat | 300 / 250 / 175 sq ft per gallon | Acrylic sealer data sheets (Behr, Rust-Oleum, Foundation Armor): 175–300 |
| Later coats | × 1.5 the first-coat coverage | The pattern in the same data sheets; site figure |
| Coats | penetrating 1, acrylic 2 | Manufacturers’ usual instructions |
| Allowance | 10% | Pump sprayer and roller loss |
| Cure before sealing | about 28 days | Concrete and mortar sealer instructions |
Assumptions last reviewed October 8, 2026.
The calculator does not model cleaning and efflorescence removal, which come first, mortar repointing, or the very high absorption of some historic soft brick, which can exceed any label. It does not choose a product for below-grade walls with water pressure, which need a waterproofing system rather than a sealer.
Two worked examples
A brick wall, penetrating sealer
20 × 15 ft of typical brick, one flood coat of a silane-siloxane sealer, 10% allowance.
- Area: 20 × 15 = 300 sq ft
- First coat: 300 ÷ 150 = 2 gallons
- Sealer: 2 × 1.10 = 2.2 gallons, buy 2 gallons + 1 quart
In metric, 6 × 4.5 m is 27 m² and 8.07 litres, a 10 L can.
An old brick patio, acrylic, two coats
300 sq ft of porous brick, a film-forming acrylic, two coats.
- First coat: 300 ÷ 175 = 1.71 gallons
- Second coat: 300 ÷ 262.5 = 1.14 gallons
- Sealer: (1.71 + 1.14) × 1.10 = 3.14 gallons, buy 3 gallons + 1 quart
The first coat takes half as much again as the second, which is why buying for one coat and doubling it overstates the job.
Where to find your inputs
Size. Length times height for a wall, length times width for a patio.
Porosity. Sprinkle water on a dry patch. If it disappears in seconds, porous; in a minute, medium; if it beads, dense (or already sealed).
Sealer type and coats. On the product label.
Coverage. The label gives a range; enter the figure for your porosity if it differs.
Common mistakes
- A second coat of penetrating sealer after the first has dried. It cannot soak in; flood the first coat instead.
- Sealing damp masonry. Moisture is trapped and acrylics blush white.
- Thick acrylic coats. Two thin coats, or it peels and whitens.
- Sealing over efflorescence. Brush and clean it off first.
- Skipping the test patch. Some sealers darken more than you expect.
- Acrylic on a walkway without grit. It is slippery when wet.
Questions people ask
- How much sealer does brick need?
- Penetrating sealers cover about 150 sq ft per gallon on typical brick, 100 on old or soft brick and 200 on dense, hard brick. A 300 sq ft wall of ordinary brick takes 2 gallons in one flood coat, 2.2 with a 10% allowance for spray and roller loss.
- Penetrating or film-forming sealer?
- Penetrating silane and siloxane water repellents soak in, leave the look of the masonry unchanged and let it breathe; they are the usual choice for brick walls and chimneys. Film-forming acrylics sit on top, darken the colour for a wet look and suit pavers, patios and decorative concrete.
- How many coats of sealer?
- Penetrating sealers usually go on as one flood coat, sometimes wet-on-wet in the same visit; a second coat after the first has dried cannot soak in. Acrylics usually take two thin coats.
- Why does the second coat use less?
- The first coat fills the open pores; the second sits on a partly sealed surface. The calculator counts later coats at one and a half times the first coat's coverage, which matches the pattern in sealer data sheets.
- When can I seal new concrete or mortar?
- After it has cured, usually about 28 days, and on a dry day after at least 24 to 48 hours without rain. Sealing damp masonry traps the moisture and can turn an acrylic white.
- Will sealer change the colour of my brick?
- Penetrating sealers usually do not; acrylics usually darken it and add a sheen. Test a hidden patch and let it dry fully before deciding.