2390
Mortar calculator
Put your wall size, bags per 100 sq ft and waste in: a 20 ft by 8 ft wall is 160 sq ft — 4.8 bags exact, 6 bags with 10% waste.
★ CONSTRUCTION · MASONRY · MORTAR
Mortar Calculator: 6 Bags for a 20×8 ft Brick Wall
On the defaults — a 20 ft by 8 ft wall, 3 bags per 100 sq ft, 10% waste — you need 6 bags of mortar. This is the mortar tool: measure the wall once, allow for the spillage, and order the bags once — a half-laid wall waits for no second delivery.
What does mortar needed 6 bags mean?
It means six bags of mortar lay a 20 ft by 8 ft brick wall once spillage and re-tempering waste is allowed for. The wall itself is 20 times 8, or 160 square feet; at 3 bags per 100 sq ft the bare count is 4.8 bags, and the 10% waste allowance takes it to 5.28, which rounds up to 6 whole bags because mortar is sold by the bag, not by the tenth. The sixth bag is not spare stock exactly — it is the mortar the job consumes in dropping, re-tempering and buttering, so a wall ordered without waste runs dry before the last course.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Wall area | length × height | 160 sq ft — 20 × 8. |
| Bags exact | wall area ÷ 100 × bags per 100 sq ft | 4.8 bags — 160 ÷ 100 × 3. |
| Bags with waste | exact × (1 + waste ÷ 100), rounded up | 6 bags — 4.8 × 1.10, rounded up. |
Three things to check before you trust the number
First, check the bag rate on the bag label: 3 bags per 100 sq ft is a planning figure for a standard brick wall, but wide joints and large blocks move it — at 4 per 100 sq ft this wall needs 8 bags, not 6. Second, subtract door and window openings from the wall area before multiplying: an opening is wall that is never mortared, and counting it over-orders by exactly its area. Third, judge the waste honestly: a mason on a straight run fits 10%, but a first-time layer, or a wall with many corners and piers, can want 15%, since every corner is dropped mortar.
Worked example
20 ft long, 8 ft high, 3 bags per 100 sq ft, 10% waste. Wall area: 20 × 8 = 160 sq ft. Bags exact: 160 ÷ 100 × 3 = 4.8. With waste: 4.8 × 1.10 = 5.28, rounded up to 6 bags. In practice that means ordering 6 bags, and expecting the last bag to go on the top courses where the re-tempering and the buttering add up. If the same wall used 4 bags per 100 sq ft instead, the need would rise to 8 bags with waste, but the wall covered would be identical.
Every branch, checked
With a 30 ft long wall instead of 20 ft, the area rises to 240 sq ft and the need to 8 bags with waste. With a 10 ft high wall, the area rises to 200 sq ft and the need to 7 bags with waste. With 4 bags per 100 sq ft, the area stays 160 sq ft but the need rises to 8 bags with waste. With 0% waste, the area stays 160 sq ft and the need falls to 5 bags. Area, bag rate and waste each move the count on their own, so a wrong bag rate is as costly as a wrong measurement.
The reverses, worked backwards
The length behind 6 bags is 20 ft: at 15 ft the need is 4 bags (120 sq ft wall, 3 bags per 100 sq ft, 10% waste); at 30 ft it is 8 bags (240 sq ft wall, 3 bags per 100 sq ft, 10% waste). The height behind 6 bags is 8 ft: at 6 ft the need is 4 bags (120 sq ft wall, 3 bags per 100 sq ft, 10% waste); at 10 ft it is 7 bags (200 sq ft wall, 3 bags per 100 sq ft, 10% waste). The bag rate behind 6 bags is 3: at 2 the need is 4 bags (160 sq ft wall, 2 bags per 100 sq ft, 10% waste); at 4 it is 8 bags (160 sq ft wall, 4 bags per 100 sq ft, 10% waste). Each reverse moves one input and holds the rest, so the step is exactly one dimension or one rate change.
What the defaults assume
The defaults assume a 20 ft by 8 ft solid brick wall face, 3 bags per 100 sq ft, and 10% waste. They do not assume door or window openings, piers, corners beyond a straight run, the bricks themselves, or sand and cement bought separately for mixing your own mortar — a wall with openings needs those areas subtracted first.
Who uses this
- A homeowner building a garden wall: the 6-bag order settles the merchant call before the first course is laid.
- A DIY builder laying a small wall: the same maths on the wall alone gives the bags, not a whole house.
- A mason quoting a job: area, bag rate, waste — the quote follows the same three steps, and the waste line shows the client why.
- A landlord budgeting a boundary wall repair: the wall size off the plans, divided by 100, gives the mortar line on the refresh budget.
Accuracy & limitations
The arithmetic is exact for the inputs entered; the judgement is the bags-per-100-sq-ft figure and the waste allowance. Real walls have openings, corners and piers, and the joint size changes the mortar per square foot; a wall measured gross over-orders, and a bag rate read off a different brick size under-orders. Treat 6 bags as the planning order for a straight 20 ft by 8 ft brick wall at 3 bags per 100 sq ft, and raise the waste to 15% for a first-time layer or a wall with many corners.
Cite this calculator
Bright Side Kit, “Mortar Calculator: 6 Bags for a 20×8 ft Brick Wall”, https://brightoolbox.com/mortar-calculator/, accessed 3 November 2026. Students and researchers: cite the calculator name, the page URL and the date you used it, and quote the defaults (20 ft × 8 ft wall, 3 bags per 100 sq ft, 10% waste) so the 6-bag result can be checked.
Frequently asked questions
How many bags of mortar do I need for a 20×8 wall?
6 bags at 3 per 100 sq ft with 10% waste. The wall is 160 sq ft; 160 ÷ 100 × 3 = 4.8 bags exact, and waste takes it to 5.28, rounded up to 6.
What does bags per 100 sq ft mean?
How many mortar bags cover 100 sq ft of wall face. This calculator uses 3 for a standard brick wall; wide joints and large blocks need more, so check the bag label.
Should I subtract door and window openings?
Yes — an opening is wall that is never mortared. Measure the wall gross, subtract each opening, and enter the net length and height, or the count over-orders.
How much waste should I allow for mortar?
10% for a straight run laid by an experienced hand, 15% for a first-time layer or a wall with many corners and piers. Waste is the spillage and re-tempering, not spare stock.
Does this count the bricks too?
No — this calculator counts mortar bags only. The brick calculator counts the bricks for the same wall, and the two orders go to the merchant together.
Pre-mixed bags or mix my own mortar?
Pre-mixed bags suit small walls: the count here is bags. For a long wall, sand plus cement can cost less per square foot, but the bag count still sizes the job — convert bags to volume before pricing bulk.