How the Concrete Mix Calculator Works

Enter your dimensions, pick a mix ratio, calculator works out the volume, applies the dry material bulking factor and splits total into each ingredient.

The bulking factor is step DIY estimates miss. Dry cement, sand and aggregate contain air voids that vanish when mixed with water. So, producing 1 cubic metre of finished concrete takes roughly 1.54 cubic metres of dry materials. The calculator handles this automatically, which is why its bag counts run higher than a naive volume split.

Which Mix Ratio for Which Job

UK site mixes are described as cement , sand, aggregate by volume or cement, ballast where all in ballast replaces the separate fines and stone:

• 1:3:6 or 1:8 ballast (GEN 1): blinding, mass fill and non structural work
• 1:2:4 or 1:6 ballast (roughly C20/GEN 3): shed bases, garage floors, strip foundations and general domestic work
• 1:1.5:3 or 1:4 ballast (roughly C25): driveways and slabs taking vehicles
• 1:1:2 (roughly C30): structural work and at this level ready mix to a designed BS 8500 specification is usually the better route

For most garden and DIY projects the 1:2:4 general purpose mix is answer, stepped up one grade where cars will drive on the result.

Ballast or Separate Sand and Aggregate

All in ballast blends sharp sand and 20mm stone in one product and for domestic work it is practical choice, one material to order, easier batching in a mixer and typically cheaper than buying components separately. A bulk bag of around 850kg produces roughly 0.4 to 0.5 cubic metres of finished concrete depending on the ratio.

Separate sand and aggregate earn their place on structural work above C25, exposed aggregate finishes where stone size shows and any job where grading control matters. Whichever route you take, use sharp sand rather than building sand, which is too fine and carries clay that weakens the mix.

Water: the Mistake That Weakens Everything

More concrete is ruined by watering can than mixer. The target water to cement ratio is 0.4 to 0.5, which means roughly 10 to 12.5 litres per 25kg bag of cement or around 150 to 160 litres per cubic metre of finished concrete. A correct mix holds its shape when squeezed in a gloved hand without crumbling, a soupy mix is dramatically weaker once cured, however easy it feels to place.

Two more habits worth keeping, mix the dry ingredients until uniform in colour before any water goes in and use each batch within two hours, never re tempering stiffened concrete with extra water.

Worked Example

A 3m x 2m shed base at 100mm thick in a 1:2:4 mix:

• Volume: 3 x 2 x 0.1 = 0.6 m³, plus 10 percent waste = 0.66 m³
• Cement: around 210kg, so nine 25kg bags
• Sharp sand: around 420kg
• 20mm aggregate: around 840kg
• Or as ballast at 1:6: around 1,370kg, so two bulk bags with spare, plus eight bags of cement
• Water: roughly 100 litres added gradually

Above 1 cubic metre, price up ready mix or a mini mix delivery before committing to hand batching, since labour saving often outweighs price difference.