How to use the mortar calculator calculator
- 1
Measure the wall face
Add a section for each wall, using the net area you are actually building. Subtract large openings, since they contain neither units nor mortar.
- 2
Enter all three unit dimensions
Face length, face height, and depth through the wall. The depth is what sets the wall thickness, and it matters as much as the face size, because it determines the wall volume.
- 3
Enter the joint thickness
This one is required rather than optional, because the joint is precisely what the mortar fills. Use the joint your mason is working to.
- 4
Add a waste allowance
The geometric result is the net joint volume. Real consumption is higher because of mixing loss, furrowing, squeeze-out, and full bedding, so choose a percentage that reflects your work.
- 5
Add a bag yield for a bag count
Enter the yield printed on the mix bag or given in its documentation to convert volume into bags. YardCalc publishes no yield for any mix.
Formula
Mortar cu ft = (wall area x unit depth) - (unit count x unit L x H x D / 1728)
The unit count comes from the wall area divided by the module area, which is the unit face plus one joint in each direction.
Worked example
A 100 square foot wall of units measuring 7.625 by 2.25 by 3.625 inches, with a 0.375 inch joint
- 1Module area: (8 / 12) x (2.625 / 12) = 0.1458 square feet.
- 2Units in the wall: 100 / 0.1458 = 686.
- 3Wall volume: 100 sq ft x (3.625 / 12) = 30.21 cubic feet.
- 4Unit volume: 686 x (7.625 x 2.25 x 3.625) / 1728 = 24.68 cubic feet.
- 5Mortar: 30.21 - 24.68 = 5.53 cubic feet.
The net joint volume is about 5.53 cubic feet, which is 18.3 percent of the wall. Those unit dimensions are the ones entered for this example, not a standard size.
Worked examples
What a thicker joint costs
The same wall with a 0.5 inch joint instead of 0.375
- 1Module area: (8.125 / 12) x (2.75 / 12) = 0.1552 square feet.
- 2Units: 100 / 0.1552 = 645, which is 41 fewer.
- 3Unit volume: 645 x 0.035992 = 23.21 cubic feet.
- 4Mortar: 30.21 - 23.21 = 7.00 cubic feet.
A joint one eighth of an inch thicker raises the mortar by 27 percent and reduces the unit count by 41. Joint thickness moves both quantities at once, in opposite directions.
Converting to bags
The 5.53 cubic feet above, with a 20 percent allowance and a 0.5 cubic foot bag yield
- 1Apply the allowance: 5.53 x 1.20 = 6.64 cubic feet.
- 2Divide by the yield entered: 6.64 / 0.5 = 13.3 bags.
- 3Round up: 14 bags.
Fourteen bags at that example yield. Both the 20 percent allowance and the 0.5 cubic foot yield are figures entered for this example; read the yield from the bag you buy.
How the geometric method works
Think of the finished wall as a solid slab: its face area multiplied by the thickness of one unit. That solid is made of two things, the units and the mortar between them, so if you can work out how much of it the units occupy, the mortar is whatever is left.
The unit count comes from the same module arithmetic used on the brick and block pages: the wall area divided by the unit face plus one joint in each direction. Multiplying that count by the volume of one unit gives the volume of masonry, and subtracting it from the wall volume gives the joints.
The appeal of this approach is that it has no hidden assumptions. Every input is a dimension you can measure, and the result follows from them exactly. Its limitation is equally clear: it describes an idealized wall where the joints are exactly the stated thickness and completely filled, and no material is wasted. Real walls differ, which is why the allowance exists.
Why joint size matters so much
The joint is a large fraction of a masonry module, particularly in the vertical direction. On a unit 2.25 inches high, a 0.375 inch joint adds 17 percent to the height of the repeating module.
That means joint thickness pushes two quantities in opposite directions at once. A thicker joint means fewer units in the same wall, and more mortar between them. The worked example above shows an eighth of an inch changing the unit count by 41 and the mortar by 27 percent.
It also means the joint you enter should be the joint being built, not a nominal figure. If the mason is working to a different thickness than the drawings assume, both the unit order and the mortar order shift.
Hollow units, and what this does not cover
The geometric method treats each unit as a solid rectangular block. For brick that is close enough. For hollow concrete block it overstates the unit volume, because part of what the calculation treats as masonry is actually void.
The practical effect is that a hollow-unit wall has more open volume than this calculation assumes. If the cores are left empty, the mortar figure is still roughly right, because mortar only fills the joints and not the cores. If the cores are grouted, the grout is an additional quantity entirely, and a substantial one.
Whether cores need grouting is a structural question rather than an arithmetic one. Calculate the joints here, and take the grout quantity from your drawings.
Common mistakes
Where mortar estimates go wrong:
- Ordering the net joint volume with no allowance, when real consumption is always higher.
- Leaving out the unit depth, which is what sets the wall volume the calculation starts from.
- Entering a nominal unit size along with a joint, which double counts the mortar.
- Using a joint thickness from the drawings when the mason is working to a different one.
- Treating a grouted hollow-block wall as a joints-only problem.
- Assuming a bag yield rather than reading it off the bag.
- Applying a published coverage rate to a wall whose units, joint, or bond differ from whatever that figure assumed.
Sources and methodology
Every figure on this page comes from geometry, not from a product or materials database. Areas use the standard area formulas, volumes are area multiplied by depth, cylinders use pi times the radius squared times the height, and unit conversions use exact factors: 12 inches to a foot, 27 cubic feet to a cubic yard, and 9 square feet to a square yard.
YardCalc publishes no material density, coverage rate, product dimension, soil swell factor, compaction factor, or price. Those vary by product, by supplier, by soil, and by region, and a figure that is right in one case is wrong in another, so the calculator asks you for the values that apply to your job. Weight estimates appear only when you supply a density.
Nothing here is engineering, safety, or code guidance. Results are planning estimates. Read how every value on the site is derived and checked in the YardCalc calculation methodology.
Frequently asked questions
- How much mortar do I need?
- Work out the wall volume from its area and the unit depth, then subtract the volume the units occupy. A 100 square foot wall of 7.625 by 2.25 by 3.625 inch units with a 0.375 inch joint gives about 5.53 cubic feet of net joint volume, before any allowance.
- How many bags of mortar per 100 bricks?
- That depends on the brick size, the joint thickness, and the bag yield, so there is no single answer. Enter your wall area, unit dimensions, joint, and bag yield above and the calculator works it out from geometry.
- Why does this not use a mortar coverage rate?
- Because coverage varies with the unit, the joint, the bond, the workmanship, and whether hollow cores are grouted. Publishing one figure would be wrong for most walls. Deriving the joint volume from geometry keeps every assumption visible.
- Is the result the amount I should order?
- No, it is the net joint volume for the dimensions you entered. Real consumption is higher because of mixing loss, furrowing, squeeze-out, and full bedding, so add your own allowance and confirm the quantity with your supplier.
- Does joint thickness change the brick count too?
- Yes, in the opposite direction. A thicker joint means more mortar and fewer units, because each unit occupies a larger module. Both quantities move together whenever the joint changes.
- Does this include grout for hollow blocks?
- No. Mortar fills the joints; grout fills the cores, and it is a much larger quantity. Whether cores need grouting is a structural decision, so take the grout quantity from your drawings.
Mortar volume is derived geometrically as the wall volume less the volume the units occupy, using the dimensions you enter. No coverage rate, unit size, joint width, bag yield, or price is published, and the result is a net joint volume rather than an order quantity. Quantities come from exact geometry using the dimensions you enter. Confirm product dimensions, installation requirements, local codes, site conditions, safety requirements, and final order quantities with the appropriate manufacturer, supplier, contractor, or professional.
Results are planning estimates. Last reviewed 2026-07-28.