How we calculate: concrete and mortar
Every formula and default figure our concrete and mortar calculators use. Each figure can be changed in its calculator to match your product. See how we calculate for the method behind all of them.
Concrete calculator
- Slab or footing volume (m³) = length × width × depth
- Post holes (m³) = holes × (hole area − post area) × depth
- With waste: m³ × (1 + waste); weight (t) = m³ × density
- Bags = weight (kg) ÷ bag size (kg), rounded up; or volume (L) ÷ the yield printed on the bag, rounded up
Ready-mix: ask your supplier about minimum loads.
We don't give mix strengths. Follow the maker's instructions for your job.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Density | 2.4 t/m3 | Typical value Not yet checked against the source | tradecalculator.co.uk m³-to-tonnes converter (checked 19 October 2026) Supplier's figure takes precedence. |
| Waste allowance | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Uneven ground and spillage. |
| Bag size | 20 kg | Common UK size Not yet checked against the source | From your product Bags of 20 and 25 kg are common. |
Concrete mix calculator
Open the concrete mix calculator
- Wet volume = concrete needed × (1 + waste); dry volume = wet × dry-volume factor
- For a 1 : s : g mix, total parts P = 1 + s + g (or 1 + r for 1 : r all-in ballast)
- Cement (m³) = dry ÷ P; cement (kg) = cement m³ × cement density; bags = kg ÷ bag size, rounded up
- Sand (m³) = dry × s ÷ P; gravel (m³) = dry × g ÷ P; tonnes = m³ × density
- Water = cement bags × the maker's litres per bag (low to high)
Mix ratios are typical mixes quoted by cement makers, not strength classes. For footings or anything structural, follow the engineer's or cement maker's specification.
Ready-mix: ask your supplier about minimum loads.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Parts all-in ballast: 1 : 5 cement to all-in ballast | 5 parts ballast to 1 cement | Our assumption Not yet checked against the source | Our choice: change it to suit your job A mix quoted by cement makers for general concrete (brief v2 G1). Check the cement maker’s or engineer’s spec; this is not a strength class. |
| Waste allowance | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Uneven ground and spillage. |
| Dry-volume factor | 1.54 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job Dry materials shrink when water is added (brief v2 G1). Researcher to check against Hanson, Tarmac or Blue Circle mixing guidance. |
| Cement density | 1440 kg/m3 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Typical planning figure with no source yet. Researcher to source it and reclassify as typical. |
| Sharp sand density | 1.6 t/m3 Sources range 1.6–1.7 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) Damp sand bulks; order by weight. |
| Gravel density | 1.6 t/m3 Sources range 1.6–1.75 t/m3 | Typical value Not yet checked against the source | agg.directory calculator; aggregatecalculator.co.uk density table (checked 19 October 2026) Gravel and pea shingle. Sources differ, so the range is shown. |
| Ballast density | 1.8 t/m3 Sources range 1.7–1.9 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) |
| Cement bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common UK cement bag. |
| Bulk bag size | 0.85 t | Common UK size Not yet checked against the source | builder.tools bulk bag figure (checked 19 October 2026) Varies by merchant; check yours. |
Mortar calculator
- Mortar per unit (m³) = ((length + joint) × (height + joint) − length × height) × width. Frogs are ignored.
- Wet mortar = units × mortar per unit × (1 + waste); dry materials = wet × dry-volume factor
- Cement (kg) = dry ÷ (1 + r) × cement density; sand (t) = dry × r ÷ (1 + r) × sand density, for a 1 : r mix
- Pre-mixed bags = wet mortar (L) ÷ the yield printed on the bag, rounded up
- Per m² of wall: units per m² = 1 ÷ ((length + joint) × (height + joint)); mortar per m² = mortar per unit × units per m², split into cement and sand as above (no waste)
The mix ratio depends on the job: check your maker's or builder's spec.
Frogs (the dip in some bricks) are ignored, so laying frog-up uses more.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Unit length: Bricks | 215 mm | Common UK size Not yet checked against the source | From your product UK standard brick (work size). |
| Unit width: Bricks | 102.5 mm | Common UK size Not yet checked against the source | From your product UK standard brick (work size). |
| Unit height: Bricks | 65 mm | Common UK size Not yet checked against the source | From your product UK standard brick (work size). |
| Unit length: Blocks | 440 mm | Common UK size Not yet checked against the source | From your product UK standard block face. |
| Unit width: Blocks | Enter from the bag, pack or tin | Common UK size Not yet checked against the source | From your product brief v2 B1: block thickness (wall width) for block mortar. Not stated in the brief; from the block. |
| Unit height: Blocks | 215 mm | Common UK size Not yet checked against the source | From your product UK standard block face. |
| Joint | 10 mm | Common UK size Not yet checked against the source | From your product Standard UK mortar joint. |
| Waste allowance | 10 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Droppings and mixing loss. |
| Mix ratio: parts sand to 1 part cement | 5 parts sand to 1 cement | Our assumption Not yet checked against the source | Our choice: change it to suit your job Check your maker's or builder's spec; the right ratio depends on the job. |
| Dry-volume factor | 1.3 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job Dry materials shrink when mixed with water. Researcher to check against cement makers' guidance. |
| Cement density | 1440 kg/m3 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Typical planning figure with no source yet. Researcher to source it and reclassify as typical. |
| Building sand density | 1.5 t/m3 Sources range 1.4–1.6 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) |
| Cement bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common UK cement bag. |
| Yield per pre-mixed bag | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product Enter the yield printed on your pre-mixed mortar bag. |
Screed calculator
- Average depth = (thinnest + thickest) ÷ 2 for a screed laid to falls
- Wet volume (m³) = area × average depth × (1 + waste)
- Sand and cement 1 : r: dry = wet × dry-volume factor; cement (kg) = dry ÷ (1 + r) × cement density; sand (t) = dry × r ÷ (1 + r) × sand density
- Pre-bagged: bags = wet volume (L) ÷ yield per bag, rounded up
- Liquid: order the wet volume; check the supplier's minimum load
Minimum thicknesses depend on the construction (BS 8204-1); over underfloor heating, follow the heating maker's cover.
Liquid screed is sold by the m³ with a minimum load: ask the supplier.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Minimum thickness for this construction: Bonded to the slab | To be confirmed from the source | Regulation or HSE figure BS 8204-1 (minimum thickness, bonded screed) Not yet checked against the source | Source to be added by the Researcher brief v2 G3: minimum thicknesses by construction type from BS 8204-1, shown as Advisory. Researcher to source. |
| Minimum thickness for this construction: Unbonded (on a membrane) | To be confirmed from the source | Regulation or HSE figure BS 8204-1 (minimum thickness, unbonded screed) Not yet checked against the source | Source to be added by the Researcher brief v2 G3. Researcher to source from BS 8204-1. |
| Minimum thickness for this construction: Floating (on insulation) | To be confirmed from the source | Regulation or HSE figure BS 8204-1 (minimum thickness, floating screed) Not yet checked against the source | Source to be added by the Researcher brief v2 G3. Researcher to source from BS 8204-1. |
| Waste allowance | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 G3 (test SC1 uses 5%). Researcher to confirm. |
| Mix: parts sharp sand to 1 part cement | 4 parts sharp sand to 1 cement | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 G3 (test SC1 uses a 1:4 sand–cement screed). Researcher to source to a cement or screed maker; check the maker's or specifier's mix. |
| Dry-volume factor | 1.3 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 G3 names factor.dry-screed as an assumption; test SC1 uses 1.3. Researcher to source to a screed or cement maker's mixing guidance. |
| Cement density | 1440 kg/m3 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Typical planning figure with no source yet. Researcher to source it and reclassify as typical. |
| Sharp sand density | 1.6 t/m3 Sources range 1.6–1.7 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) Damp sand bulks; order by weight. |
| Cement bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common UK cement bag. |
| Bulk bag size | 0.85 t | Common UK size Not yet checked against the source | builder.tools bulk bag figure (checked 19 October 2026) Varies by merchant; check yours. |
Screed drying time calculator
Open the screed drying time calculator
- Sand and cement rule of thumb: 1 day per mm up to 40 mm, then 2 days per mm beyond (our adjustable defaults)
- Days = thickness up to the threshold × first rate + thickness beyond × second rate
- Anhydrite, fast-drying and other screeds: days = thickness (mm) × the maker's days per mm
A rule of thumb, not a test. Measure moisture before laying (hygrometer or the method the floor-finish maker asks for).
Cold, damp or unventilated rooms dry more slowly.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Rule of thumb: days per mm, up to the threshold | 1 days per mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 G4: rule of thumb commonly quoted for sand–cement screed, 1 day per mm up to 40 mm. Researcher to source to a screed maker or BS 8204 guidance and state it's a rule of thumb. |
| Rule of thumb: threshold thickness | 40 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 G4: the rule of thumb changes rate after 40 mm. Researcher to source. |
| Rule of thumb: days per mm beyond the threshold | 2 days per mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 G4: 2 days per mm beyond 40 mm (rule of thumb). Researcher to source. |