How the mortar calculator works
Mortar quantities are estimated from the number of units being laid. With 10mm joints, a standard brick uses about 0.9kg of dry mortar (sand + cement) and a 100mm block about 2kg. The total is then split according to your mix ratio. Lime is light (about 0.6kg per litre compared with 1.44kg for cement), which the calculator allows for.
Mortar mix guide
| Mix | Strength | Use for |
|---|---|---|
| 3:1 | High | Below DPC, copings, retaining walls, chimneys |
| 4:1 | Medium-high | General external brickwork |
| 5:1 / 6:1 | Medium | Sheltered and internal walls, blockwork |
| 1:1:6 lime | Medium-low | Soft bricks, older properties, flexible joints |
A typical mixer load is about 1 bag of cement with the matching sand. The calculator shows how many mixes your job needs so you can plan your day.
Worked example: 1,000 bricks in a 4:1 mix
1,000 bricks × 0.9kg = 900kg of dry mortar, plus 10% wastage = 990kg. Split by weight for a 4:1 mix, that's about 808kg of building sand and 182kg of cement. Order 8 × 25kg bags of cement (7.3 rounded up) and one 850kg bulk bag of sand. That works out at roughly one bag of cement per 135–140 bricks, which matches the usual rule of thumb on site.
In a 1:1:6 lime mix the same 1,000 bricks need about 5 bags of cement, 3 bags of hydrated lime and 820kg of sand. The lime makes the mix softer, more workable and kinder to old bricks.
Not sure how many bricks you need? The brick calculator works out bricks and mortar together from the wall size, and the block calculator does the same for blockwork.
Mixing good mortar
- Gauge by bucket, not shovel. Shovel sizes vary with how wet the sand is. A bucket of cement to four buckets of sand gives the same mix and colour every time.
- Use plasticiser, not washing-up liquid. A proper mortar plasticiser adds tiny air bubbles that improve workability and frost resistance without weakening the mix.
- Add water gradually. Mortar should hold its shape on the trowel and slide off cleanly, not slump. Too much water weakens it and stains the bricks.
- Use it within about two hours. Throw away mortar that has started to stiffen. Adding water to bring it back ("knocking up") reduces its strength.
- Keep the sand consistent. Buy all the sand for one wall from the same supplier and batch. Different sands give visibly different joint colours.
Weather and curing
Don't lay bricks when the temperature is below about 3°C and falling, or when frost is forecast before the mortar has set. Cover new work overnight with hessian or a sheet, keeping it off the face of the bricks. In hot, windy weather, dampen very dry bricks or blocks lightly so they don't suck the water out of the mortar too fast.
Common mortar mistakes
- Mortar stronger than the bricks. With soft or reclaimed bricks, a 3:1 cement mortar can cause the bricks to crack and spall. Choose a weaker or lime mix.
- Sharp sand in bricklaying mortar. It makes a harsh, unworkable mix. Use soft building sand.
- Under-ordering. Filled frogs, thick joints and blocks laid flat all use more mortar than the averages. Keep the 10% wastage allowance.
For a full walk-through of a garden wall, from footing to coping, read our garden wall guide, or the brick and mortar quantities guide for figures per square metre.