How to Calculate Mortar for a Brick Wall in 5 Steps

Mortar for a brick wall in one line

Mortar for a brick wall is wall area, divided by 100, times bags per 100 sq ft, plus waste. A 20 ft by 8 ft wall is 160 square feet; at 3 bags per 100 sq ft that is 4.8 bags exact, and with 10% waste the order is 6 bags. The rest of this guide is about getting those three inputs honest: the wall size, the bags per 100 sq ft, and the waste allowance.

Step 1: Measure the wall face

Measure the length and the height of the wall face in feet, edge to edge, on the face itself — not the footing, since the mortar count follows the face area, and a wall face is what the bricks cover. The worked example is 20 ft long and 8 ft high, a normal garden wall. Walk the tape along the face, not around the back, and if the wall has a door or window opening, measure that opening too: it comes off the area in step two, since an opening is wall that is never mortared. A wall that is 20 ft by 8 ft has an area of 160 square feet, and that one number is the foundation of everything that follows.

Step 2: Divide the area by 100

Mortar is rated by the hundred square feet, so divide the wall area by 100: 160 divided by 100 is 1.6 hundreds of square feet. This division is the step that turns a measurement into a bag rate, and it is where openings are subtracted: a 20 ft by 8 ft wall with a 3 ft by 7 ft door is 160 minus 21, or 139 square feet, which is 1.39 hundreds. Keep the decimal — it is not rounded away, since a fraction of a hundred square feet is still a fraction of a bag.

Step 3: Multiply by bags per 100 sq ft

Multiply the hundreds by the bags per 100 sq ft from the bag label. A standard brick wall takes 3 bags per 100 sq ft, so 1.6 hundreds times 3 is 4.8 bags exact. Wider joints and larger blocks take more — at 4 bags per 100 sq ft the same wall needs 6.4 bags exact, a whole third more, from one label figure. Check the label before multiplying, not after ordering: the bag rate is the input people assume instead of reading, and the assumption is what under-orders.

Step 4: Add waste for spillage

Mortar is dropped, re-tempered and buttered, so add waste before ordering: 10% for a straight run takes 4.8 bags to 5.28. A first-time layer, or a wall with many corners and piers, wants 15%, which would take the same wall to 5.52 bags before rounding. Waste is not spare stock — it is the mortar the job consumes in dropping and re-tempering, and a wall ordered without it runs dry before the last course, with the merchant closed and the wall open.

Step 5: Order whole bags, and count the extras

Order 6 bags, since mortar is sold by the bag and 5.28 rounds up, not down. Then count what bags do not cover: the bricks themselves, the sand and cement if you mix your own, the water, and the tools — none of these come in the bag count. Keep one spare bag after the job: it patches a cracked joint, and a matching batch may not exist next year.

Mortar in numbers

Suppose the wall is 20 ft by 8 ft, at 3 bags per 100 sq ft, with 10% waste. Area: 20 × 8 = 160 sq ft. Hundreds: 160 ÷ 100 = 1.6. Bags exact: 1.6 × 3 = 4.8. With waste: 4.8 × 1.10 = 5.28, rounded up to 6 bags. Order: 6 bags, plus bricks bought separately.

Common mistakes

The most common mistake is measuring the wall gross and forgetting the openings, which over-orders by exactly the opening area. The second is assuming 3 bags per 100 sq ft without reading the label, when wide joints take 4. The third is skipping the waste allowance, which under-orders by exactly the amount the dropping and re-tempering consume. The fourth is counting the bricks as mortar, which are bought by the piece and counted separately. The fifth is rounding down instead of up, which orders a wall short by the last course.

Worked example, start to finish

A wall, 20 ft by 8 ft, 3 bags per 100 sq ft, 10% waste. Area: 160 sq ft. Hundreds: 1.6. Bags exact: 4.8. With waste: 5.28, rounded up to 6 bags. Result: order 6 bags, plus bricks bought separately, and keep one spare bag for joint repairs. If the same wall used 4 bags per 100 sq ft instead, the exact count would be 6.4 bags and the order 8 with waste — the bag rate alone would add 2 bags, which is why step three, reading the label, matters most.

What changes the answer most

The bags-per-100-sq-ft figure changes the answer most: moving from 3 to 4 adds a third to the whole order at a stroke, since it multiplies every hundred square feet. The wall size is second: length and height move the area, but only across a bag rounding does the shop list change. The waste allowance is third: 10% is small beside the bag rate, but it is the difference between a finished top course and an open one. The openings are fourth, and they change only the measured area, never the formula. If the budget is tight, build a shorter wall before cutting the waste allowance.

Frequently asked questions