Mortar Mix Design Calculator
Trial strength, cement, water and sand, and the mix by mass.
Inputs
Result
Formula
| Trial strength | fm,o = k × f2 with k from 1.00 to 1.25 |
| Cement | Qc = 1000(fm,o + 15.09) ÷ (3.03 fce) |
| Water | Qw = Qc × w/c |
| Sand | Qs = (1000 − Qc/3.10 − Qw) × ρs |
| Mix by mass | cement : sand = 1 : Qs/Qc |
| k values | 1.25 for M5-7.5, 1.20 for M10, 1.15 for M15 |
M10 mortar with 42.5 cement at w/c 0.65 and 1.45 kg/L sand: trial strength 12.00 MPa, 210 kg/m³ of cement, 137 kg of water and 1153 kg of sand — a 1:5.49 mix, or 274 kg of sand per 50 kg bag.
FAQ
Why not size mortar with a water-cement ratio formula?
Because mortar has no coarse aggregate, so its strength comes from the cement paste binding the sand — a different mechanism from concrete, where Bolomey's formula would be far off. JGJ/T 98 uses a statistical regression tying cement strength directly to the target, which matches trial results better on site. Trials still govern.
Is a 1:3 volume mix good enough?
Workable, but understand that its strength is uncertain. Volume batching lets the cement content swing between 350 and 450 kg/m³ as the sand bulks and wets, which is a whole grade of strength. Proper practice is weighing — especially for load-bearing masonry and large plaster areas, where volume batching is the main cause of scatter.
Does higher cement grade mean less cement?
Yes, but only down to a floor — usually 200 kg/m³. Below that the mortar loses workability and water retention: it will not hang on a trowel or a block, and plaster crazes. So a 52.5 cement for M5 computes at 120 kg but must be raised to 200 with fly ash or lime to restore workability, or the saving costs the quality.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.