Calculator — Concrete & Masonry
Cement Calculator
Bags of cement plus sand and aggregate volumes for site-mixed concrete — dry volume, mix ratio and bag size included.
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Results
Adjust the inputs to see your result.
About this calculator
Site-mixed concrete starts from dry volume: wet concrete shrinks as it fills the voids between particles, so dry ingredients total 1.54 times the finished volume. This calculator splits that dry volume by your mix ratio into cement bags, sand and aggregate.
What this number means
This calculator answers the question ready-mix buyers never ask: how many bags of cement does my slab actually contain? The trick is the dry-volume factor. Wet concrete shrinks as it fills the voids between sand and aggregate particles, so the dry ingredients total about 1.54 times the finished volume — a slab that pours 40 cu ft needs 61.6 cu ft of dry material, not 40.
From there the mix ratio splits the dry volume: in a 1:2:4 mix, one part in seven is cement. Cement weighs about 94 lb per cubic foot, which is conveniently the weight of a standard US bag — so the cement volume in cubic feet is close to the bag count before rounding. Sand and aggregate volumes come from the same split, so one calculation produces the whole materials list.
The formula
- Wet volume = Length × Width × Depth ÷ 12 (cu ft)
- Dry volume = Wet volume × 1.54
- Cement volume = Dry volume × cement parts ÷ total parts
- Cement weight = Cement volume × 94 lb per cu ft
- Bags = Cement weight ÷ bag weight, rounded up
- Sand volume = Dry volume × sand parts ÷ total parts; Aggregate volume = Dry volume × aggregate parts ÷ total parts
Worked example
A 12 × 10 ft patio slab, 4 in thick, 1:2:4 mix, 94 lb bags.
- Wet volume: 12 × 10 × 4 ÷ 12 = 40 cu ft (1.48 cu yd)
- Dry volume: 40 × 1.54 = 61.6 cu ft
- Cement: 61.6 × 1 ÷ 7 = 8.8 cu ft → 8.8 × 94 = 827.2 lb → 9 bags
- Sand: 61.6 × 2 ÷ 7 = 17.6 cu ft
- Aggregate: 61.6 × 4 ÷ 7 = 35.2 cu ft
Assumptions and limitations
- The 1.54 dry-volume factor is the standard allowance for voids between aggregate particles; very fine or very coarse aggregates shift it slightly.
- Cement density is taken as 94 lb per cubic foot, the standard figure behind the 94 lb bag.
- Water is not counted as a purchased material here — roughly half the cement weight in water is typical for workable mixes, but it comes from a hose, not a bag.
- Waste and spillage are not included; add 5–10% to the bag count for a first-time mix or an uneven subgrade.
- For large pours, ready-mix by the cubic yard is usually cheaper than bagged cement — compare with the Concrete Calculator before ordering.
Frequently asked questions
Wet volume is 33.3 cu ft, dry volume 51.3 cu ft. In a 1:2:4 mix the cement share is 7.3 cu ft (689 lb), so 8 × 94 lb bags — plus a spare bag for spillage and an uneven subgrade.
Dry ingredients occupy more volume than wet concrete because sand and aggregate particles leave voids between them. Multiplying the finished (wet) volume by 1.54 gives the dry volume you must actually buy and mix.
Use 1:2:4 for general slabs, patios and driveways. Use 1:1.5:3 for structural work and footings where strength matters. Use lean 1:3:6 only for blinding layers and mass fill that carries no load.
One 94 lb bag yields about 4.5 cu ft of 1:2:4 concrete — roughly a 3 × 3 ft patch at 4 in thick, or about 0.17 cu yd. For anything over a cubic yard, ready-mix is usually the cheaper route.
They come from the same dry-volume split as the cement. For the worked example (12 × 10 ft, 4 in, 1:2:4): 17.6 cu ft of sand and 35.2 cu ft of aggregate alongside the 9 bags of cement.
The Concrete Calculator gives cubic yards for a ready-mix order. This Cement Calculator breaks a job into bags of cement plus sand and aggregate volumes for site mixing. Use this one when you are mixing by hand or in a mixer; use the other when a truck is delivering.