Mortar Calculator
A mortar calculator estimates 80-lb bags of mortar mix from wall area at about 25 sq ft per bag: a 20 × 8 ft block wall takes 7 bags, or $63 at $9 each.
Mortar calculator: bags for a block wall
A mortar calculator divides your wall area by the coverage of one bag and rounds up to whole bags. This one assumes that an 80 lb bag of mortar mix lays about 25 sq ft of standard concrete block wall, and it prices the bags at $9 each by default.
Bags = wall area (sq ft) ÷ 25, rounded up. Cost = bags × price per bag.
The area is length times height. For a wall 20 ft long and 8 ft high, the area is 160 sq ft.
How the coverage figure works
A block wall is mostly block with thin mortar joints between them. A standard block face is 16 in by 8 in, which is about 0.89 sq ft, so a 160 sq ft wall holds around 180 blocks. The mortar fills the bed joints under each block and the head joints between them, at about 3/8 in thick. The 25 sq ft per bag figure is a planning average. Thicker joints, wet mixes, and spills lower it.
Worked examples
Example 1, the default wall. 20 ft × 8 ft, $9 per bag.
- Area: 20 × 8 = 160 sq ft
- Bags: 160 ÷ 25 = 6.40, rounded up to 7
- Cost: 7 × $9 = $63
Example 2, a long garden wall. 30 ft × 6 ft.
- Area: 180 sq ft
- Bags: 180 ÷ 25 = 7.20, rounded up to 8
- Cost: 8 × $9 = $72
Example 3, an exact fit. 10 ft × 10 ft.
- Area: 100 sq ft
- Bags: 100 ÷ 25 = 4.00, which stays at 4
- Cost: $36
Bags by wall size
| Wall (length × height) | Area | Bags | Cost at $9 |
|---|---|---|---|
| 16 × 4 ft | 64 sq ft | 3 | $27 |
| 10 × 10 ft | 100 sq ft | 4 | $36 |
| 20 × 8 ft | 160 sq ft | 7 | $63 |
| 30 × 6 ft | 180 sq ft | 8 | $72 |
| 40 × 8 ft | 320 sq ft | 13 | $117 |
Walls with large openings need a smaller area. Subtract doors and windows from the wall area before you enter it.
Mortar types in brief
Mortar comes in letter types that run from strongest to weakest: M, S, N, O, and K.
| Type | Common use |
|---|---|
| M | Heavy loads and below-grade work |
| S | Below grade, retaining walls, areas with lateral loads |
| N | General above-grade walls |
| O | Light, non-load-bearing interior work |
The right type depends on the wall and the engineer’s or architect’s notes. Do not guess for a load-bearing or retaining wall.
What a bag of mix includes
Bagged mortar mix already holds cement, lime, and sand. You add clean water and mix to a consistency like soft butter that sticks to a trowel. Mix only as much as you can use in about an hour or so, depending on heat and wind. Retempering a little with water is common, but mortar that has begun to set should be discarded.
Measuring the wall
Measure the length of each wall run and its height from the top of the footing to the top course. Multiply, then add up separate runs. Count only the face that gets blocks, not both sides of a thin wall. Subtract the area of door and window openings.
Common mistakes
- Using the brick wall area with the block coverage figure.
- Forgetting the grout needed to fill block cores in a reinforced wall. That is separate from the mortar.
- Buying a single bag for a project that needs several, and then running out mid-course.
- Letting mortar dry out in hot weather. Dampen the block first and cover the work.
Weather and timing
Mortar cures by reacting with water, so it needs to stay damp. In hot sun or wind, wet the block first and mist the finished wall under a cover for a day or two. In cold weather, mortar can freeze before it gains strength, and masons use warm water or heated enclosures. Check the bag for its temperature limits.
Related calculators
The concrete block calculator counts blocks for the same wall, and the brick calculator does the same for brick. For tile work, the thinset calculator covers a different bag. Retaining or load-bearing walls should be checked against your local code or by a licensed professional.
Frequently asked questions
How much wall does one bag of mortar cover?
This calculator assumes one 80 lb bag lays about 25 sq ft of standard concrete block wall. A 160 sq ft wall divides to 6.4 bags, and the page rounds up to 7. Joint thickness, how wet you mix, and how much falls off the board all change the real number.
Type N or Type S mortar: which do I need?
Type N is a general-purpose mix for above-grade walls. Type S is stronger and is often called for below grade, in retaining walls, and where wind or soil pushes sideways on the wall. Your plans or the local building department name the type, and the mix itself costs about the same.
Does this calculator work for brick walls too?
Only roughly. Brick is smaller than block, so a square foot of brick wall has more joints and uses more mortar than a square foot of block. Use the brick calculator for brick, and treat this tool as a block-wall estimate.
Does the calculator round the bag count up?
Yes. Bags are sold whole, so 6.4 becomes 7, and the cost uses 7 bags. If your wall is an exact multiple, such as 100 sq ft, it stays at 4 bags with no extra.
Should I add extra mortar for a first-time DIY wall?
It is a good idea. Beginners drop and waste more mortar, and mixed mortar that sets in the tub is a total loss. Many people buy one bag more than the estimate, since a spare unopened bag can usually go back or be saved.
Estimates only. Verify quantities with your supplier before purchasing.