DIY Concrete Mix Calculator (Bags, Sand, Gravel & Water)
Quickly work out cement bags, sand, gravel, water and additives for hand mixing.
DIY Concrete Mix Calculator Guide (UK 2025)
This guide explains how the DIY Concrete Mix Calculator turns your project volume into exact quantities of cement bags, sand, gravel and water for hand mixing. It supports shape inputs (rectangle, triangle, circle) or known volume, plus optional additives and a water–cement ratio.
🧰 Materials & Tools for Mixing Concrete
- • Blue Circle General Purpose Grey Cement in Paper Bag 25kg
- • Sand and Stone Ballast Bulk Bag - Travis Perkins
- • Hire Electric Cement Mixer-Travis Perkins
- • Concrete Plasticiser / Additive Sika 4 Rapid Setting Liquid Admixture 5L
- • Concrete Reinforcement Merchant Steel Metal Fabric Mesh 2.42m x 1.22m
- • Ram 18'' Plastering Trowel-Travis Perkins
- • Wood Float Trowel-Travis Perkins
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How the Calculator Works
Why 1.54? Aggregates have voids and batching by shovel/bucket introduces loss. Multiplying wet volume by ~1.54 gives a realistic dry-batched volume for cement + sand + gravel.
Typical Mix Ratios & Use Cases
| Mix (C:S:G) | Use | Notes |
|---|---|---|
| 1 : 2 : 3 | General purpose slabs, pads, paths | Good all-rounder for hand mixing. |
| 1 : 2 : 4 | Light-duty slabs, garden works | More aggregate, slightly leaner. |
| 1 : 1.5 : 3 | Higher strength or thinner sections | Richer in cement; watch w/c to limit shrinkage. |
Assumptions & Densities (Editable in Calculator)
| Parameter | Default | What it means |
|---|---|---|
| Dry Factor | 1.54 | Converts wet concrete volume to dry-batched volume of all constituents. |
| Water–Cement Ratio (w/c) | 0.50 | Water mass divided by cement mass (L ≈ kg). Lower = stronger but stiffer. |
| Cement Density | 1440 kg/m³ | Bulk density used to turn cement volume to mass. |
| Sand Density | 1600 kg/m³ | Varies with moisture and grading (1500–1700 typical). |
| Gravel Density | 1500 kg/m³ | Assumed for 10–20 mm aggregate. Adjust per supplier. |
| Cement Bag Size | 25 kg | Set to 25 kg, 20 kg, or your preferred bag. |
| Additive % (optional) | 0% | Simple estimate: additive volume as % of water. Always follow datasheet. |
Example Calculation
Slab: 3.5 m × 2.5 m × 100 mm (0.10 m), mix 1:2:3, w/c 0.5, bag 25 kg.
Always add water slowly and stop when the mix holds a formable shape without slumping.
Average UK DIY Material Prices (2025)
| Item | Typical UK Price (ex. VAT) | Notes |
|---|---|---|
| Cement 25 kg | £5.50 – £8.50 | Brand and region vary; multi-buy often cheaper. |
| Sharp sand (bulk bag ~850 kg) | £35 – £60 | Check delivery charge and access. |
| 10–20 mm gravel (bulk bag ~850 kg) | £40 – £70 | Washed aggregate preferred for clean mixes. |
| Plasticiser (1–5 L) | £7 – £25 | Dose per datasheet; do not over-add. |
| Polypropylene fibres (per pack) | £10 – £25 | Helps reduce shrinkage cracking. |
| Wheelbarrow, shovel, mixing tub | £30 – £120 | Sturdy kit makes life easier. |
Tips, Workability & Curing
- Keep w/c as low as practical. If it feels too stiff, add a splash of water or use a plasticiser.
- Pre-wet your mixing tub so it doesn’t steal water from the mix.
- Place and compact promptly. Strike off, then float/finish as bleed water subsides.
- Cure: cover with polythene or use a light mist to prevent rapid drying for at least 3–7 days.
Related Calculators
Frequently Asked Questions
What is a common DIY concrete mix ratio?
Use 1:2:3 (cement : sand : gravel) for general work. For higher strength or thinner sections, try 1:1.5:3. Keep the water–cement ratio around 0.45–0.55.
How much water do I add?
Start with w/c ≈ 0.5. For each 25 kg cement bag that’s roughly 12–13 L of water. Add gradually to reach workable, not runny.
Why multiply by 1.54?
It adjusts the finished wet volume to a realistic dry-batched volume of cement, sand and gravel (voids + handling losses).
Can I add plasticiser or fibres?
Yes. Plasticiser improves workability at lower water; fibres help reduce shrinkage cracking. Always dose to the product label.
How many bags of cement do I need?
The calculator converts your dimensions to wet volume, applies the dry factor, splits by parts and densities, then divides cement mass by your chosen bag size.
