Mortar Mix Design Calculator
Trial strength, cement, water and sand, and the mix by mass.
Quick answer
With the default inputs: Mortar grade 10 MPa · Measured cement strength 42.5 MPa · Water-cement ratio 0.65 etc.Cement content 210 kg/m³
Edit the inputs below to recalculate.Inputs
Result
Cement content
210kg/m³
| Trial strength | 12.00MPa |
| Strength factor k | 1.20 |
| Water | 137kg/m³ |
| Sand | 1153kg/m³ |
| Water-cement ratio | 0.65 |
| Cement : sand by mass | 1 : 5.48 |
| Sand per 50 kg bag | 274kg |
| Mortar per 50 kg bag | 0.238m³ |
| Sand by mass in the mix | 84.6% |
The formula Qc = 1000(fm,o + 15.09) ÷ (3.03 fce) is the JGJ/T 98 regression, with 3.03 and β = −15.09 as given — do not adjust them. The k factor runs from 1.25 down to 1.00 as the grade rises, because assurance requirements are easier to meet at higher strength. Any computed mix must still be verified by trial batches.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
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.
How to work it out
- Compute the trial strength first: fm,o = fm,k + 1.645σ, with σ taken from the site's control level, normally 2.5 MPa and 3.0 where tighter. For M10 that gives 10 + 1.645 × 2.5 = 14.1 MPa.
- Actual cement strength fce = γc × fce,k with γc = 1.0, so a 42.5 grade gives 42.5 MPa.
- Cement content by the JGJ/T 98 equation: Qc = 1000(fm,o − β) / (α × fce). For masonry cement mortar α = 3.03 and β = −15.09; for cement-lime mortar α = 1.50 and β = −15.09.
- Sand: Qs = 1 − Qc/ρc with ρc the bulk density of cement at 1200-1300 kg/m³, then multiply by the sand bulk density of 1450-1550 kg/m³.
- Set water by the water-cement ratio, then trial batch and adjust. The calculation is a starting point, not the answer.
Where it goes wrong
- Using the design strength as trial strength — design strength is a 95% confidence lower bound, so without the 1.645σ margin half the batches fail.
- Setting σ to zero — no site has zero scatter, and the code's own floor is 1.25-2.5.
- Calculating with 32.5 cement while using 42.5 — the cement content comes out 30% high, giving oversize strength, more shrinkage and wasted money.
- Taking the computed cement content as final — the code caps it near 550 kg/m³ and warns below 200 kg/m³, where water retention and strength both fall short.
Material and cost
M5 masonry mortar 280-380 CNY/m³, M10 350-480, M15 420-560. Bagged dry mortar sells by the tonne at 15-25% more than site mixing but with consistent quality.
On site
- Mortar cubes are 70.7 mm, cured 28 days — a different specimen from the 150 mm concrete cube, so the strengths are not comparable.
- Sand with over 5% silt sharply reduces strength by weakening the cement-sand bond; check the silt content on delivery.
- Lime putty or fly ash improve workability and cut cement, but they lower early strength and need care in winter.
Basis
- JGJ/T 98-2010 Specification for mix proportion design of masonry mortar
- GB 50203-2011 Code for acceptance of masonry construction, Chapter 4
- GB/T 25181-2019 Ready-mixed mortar
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Mortar Mix Design Calculator. 2026-10-25. https://jianzhujisuan.com/en/mortar-strength
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.