Batch concrete or mortar from raw materials: enter a volume and a mix ratio, and read off cement in bags, sand and gravel by volume and weight, and the water — with the dry-volume factor and the water-cement ratio already built in, so your order doesn’t come up short.
Mix type
Units
Volume of finished mix
Length (ft)
Width (ft)
Thickness (in)
A nominal volumetric mix. Above about M20, use a designed mix from a supplier rather than a fixed ratio.
0.4–0.6 typical.
Advanced: dry factor & densities
1.54 concrete, 1.30 mortar.
Material-proportioning estimate from standard site-estimation formulas, nominal volumetric ratios and typical material densities — all editable. Mix design, reinforcement, admixtures and finishing are out of scope. Real sand and aggregate vary with moisture and grading, so treat the output as an order estimate, not a structural specification.
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Buying bags, setting posts, or ordering the gravel?
If you are buying pre-blended bags rather than mixing your own, the Concrete Calculator turns a volume straight into ready-mix bags and cost. Setting fence or deck posts? The Fence Post Concrete Calculator works out the concrete per hole. And the loose sand or gravel for the mix — or a sub-base under a slab — comes out in tons and yards in the Gravel Calculator.
What this concrete mix ratio calculator does — and its lane
This is a raw-material proportioning tool. You tell it how much finished concrete or mortar you need and the ratio you want to mix at, and it works out how much loose cement, sand and aggregate to buy, plus the water to add. That is a different job from buying pre-blended bags: a ready-mix bag already has the cement, sand and stone combined, so for those you only need a volume divided by a bag yield, which the concrete calculator handles. Use this page when you are batching from separate materials — the classic cement, sand and gravel pile and a mixer.
From volume to cement, sand, aggregate and water
The chain of arithmetic is short but easy to get wrong, and the calculator shows each step. The crucial move most people miss is the dry-volume factor: loose dry materials hold a lot of air between the grains, so a heap of them shrinks by about a third once mixed and compacted. To finish with a cubic metre of placed concrete you therefore have to start with roughly 1.54 cubic metres of dry stuff. Skip that step and your order lands about 35% short.
dry volume = wet volume × dry factor × (1 + waste÷100) each material = dry volume × (its parts ÷ total parts) cement mass = cement volume × 1440 kg/m³ · bags = cement mass ÷ bag weight water (litres) = cement mass × water-cement ratio
From there, cement volume is turned into mass at about 1440 kg per cubic metre and then into whole bags. Sand and aggregate are reported both as a volume — in cubic feet, cubic yards or cubic metres — and as a weight, using typical bulk densities of about 1600 kg/m³ for sand and 1550 kg/m³ for coarse aggregate, which you can change if your supplier quotes different figures. The mixing water follows from the water-cement ratio, shown in both litres and US gallons.
Mix ratios, nominal strength and the water-cement ratio
The presets cover the everyday nominal mixes. Leaner mixes like 1:4:8 and 1:3:6 suit blinding and mass fill; 1:2:4 is the general-purpose workhorse; and 1:1.5:3 is a stronger nominal mix. The approximate M-band shown next to each is a guide only — once you need something stronger than about M20, a fixed volumetric ratio stops being reliable and you want a designed mix that accounts for the actual cement, aggregate grading and exposure conditions. Swap to the mortar tab for a cement-and-sand-only blend; it uses the lower 1.30 dry factor because mortar packs tighter than concrete.
Nominal ratio
Total parts
Typical use
Approx. band
1 : 1.5 : 3
5.5
Slabs, beams, columns
≈ M20
1 : 2 : 3
6
Stronger general work
≈ M20
1 : 2 : 4
7
General-purpose
≈ M15
1 : 3 : 6
10
Foundations, mass fill
≈ M10
1 : 4 : 8
13
Blinding, lean fill
≈ M7.5
The water-cement ratio is just the weight of water over the weight of cement. Lower numbers give stronger, stiffer concrete; higher numbers are easier to place but weaker, and past about 0.6 the strength drops off noticeably. The default here is 0.5. Bear in mind damp sand already carries water into the mix, so trim the added water a little when the sand is wet.
US and UK terms, metric and imperial
People say cement and concrete, aggregate and gravel, metre and meter for the same things, and order by the cubic yard or the cubic metre depending on where they are. This calculator takes either unit system and reports volumes in ft³, yd³ and m³ together, and weights in kg, tonnes and lb, so you can hand a metric figure to one supplier and an imperial one to another without converting anything by hand.
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Frequently asked questions
How do I calculate cement, sand and aggregate for a mix ratio?
Take the wet volume you need, multiply by the dry factor 1.54, then split that dry volume by the parts of your ratio. For 1:2:3 (6 parts) cement is 1/6, sand 2/6, aggregate 3/6. Turn the cement volume into mass at ~1440 kg/m³ and divide by your bag weight for the bag count. This tool does it live and adds the water.
Why multiply by 1.54 — what is the dry-volume factor?
Dry cement, sand and aggregate hold air between the grains; once mixed and compacted the batch shrinks by about a third. So one cubic metre of placed concrete needs about 1.54 m³ of dry materials. Forgetting this is the top reason people under-order and run short. Mortar packs tighter, so it uses ~1.30 instead.
How is this different from a ready-mix bag calculator?
This proportions loose cement, sand and aggregate that you mix yourself. A ready-mix bag already blends all three, so it just needs volume ÷ bag yield — that is the job of the concrete calculator. Mixing your own from separate piles? You are in the right place.
How much water do I add per bag of cement?
Water is the water-cement ratio × the cement weight. At 0.5, a 50 kg bag takes about 25 litres. Most concrete sits between 0.4 (stronger, stiffer) and 0.6 (easier, weaker); above ~0.6 it weakens noticeably. Wet sand carries water too, so cut the added water a little on a damp day.
What ratio should I use, and how does it map to M-grades?
Roughly: 1:4:8 and 1:3:6 are lean fill (≈M7.5–M10), 1:2:4 is general-purpose (≈M15), 1:1.5:3 is stronger (≈M20). The bands are approximate. Above about M20, drop the fixed ratio and use a designed mix from a supplier — the labels here are generic guidance, not a code table.
Can I change the densities and bag size?
Yes. Cement is ~1440 kg/m³, sand ~1600, aggregate ~1550 — typical averages you can overwrite. Bag weight switches between 50 kg, 40 kg and 94 lb. Enter your own numbers and everything recalculates. It is an order estimate, so add a little for what stays in the mixer.