Mortar Mix Ratio Calculator
Scale exact batch volumes of Portland cement, hydrated lime, and washed masonry sand for ASTM C270 Type M, S, N, and O mortars.
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ASTM C270 Standard Proportions by Volume
Mortar binds bricks and blocks while cushioning dimensional irregularities. Never use pure concrete or high-strength Type M on historic or soft clay bricks, as mortar must always be weaker than the brick to absorb thermal movement.
- Type M (1 : 0.25 : 3): 1 part Portland, 1/4 part Lime, 3 parts Sand. Highest compressive strength for soil contact and manholes.
- Type S (1 : 0.5 : 4.5): 1 part Portland, 1/2 part Lime, 4-1/2 parts Sand. High flexural tensile bond strength for block foundations.
- Type N (1 : 1 : 6): 1 part Portland, 1 part Lime, 6 parts Sand. Universal general purpose for above-grade exterior walls and chimneys.
- Type O (1 : 2 : 9): 1 part Portland, 2 parts Lime, 9 parts Sand. High lime content for soft historic repointing and tuckpointing.
Frequently Asked Questions
Why is hydrated lime added to mortar?
Lime increases workability (making mortar buttery and stick to the trowel), increases water retention, and provides "autogenous healing" where microcracks reseal when exposed to rainwater and atmospheric CO2.
What happens if mortar is too strong for the brick?
If mortar is stronger than the brick (e.g. using Portland cement on historic pre-1930 soft clay bricks), thermal expansion causes the brick face to spall and crack instead of the sacrificial mortar joint.
How much water should be added to the batch?
Add clean potable water gradually until the mortar clings to an inverted mason’s trowel without sliding off, but spreads smoothly with light pressure.