MinuteMetric

How Much Concrete Do You Need?

Concrete is sold by volume, and volume is where most DIY projects go wrong. Order too little and you're stuck with a cold joint where the first pour has started to cure before the second arrives — a permanent weak line through your slab. Order too much and you've paid for material you have to dispose of. This calculator converts your dimensions into cubic feet, cubic yards and the number of pre-mixed bags, so you can price the job either way.

How to Use the Calculator

1. Choose your units: Imperial (feet and inches) or metric (metres and centimetres).
2. Pick a shape: Slab or rectangle for floors, patios and driveways; footing for strip foundations and trenches; column or cylinder for posts and piers.
3. Enter the dimensions: Length and width in feet or metres, and depth in inches or centimetres — depth is entered in the smaller unit because slabs are usually only a few inches thick.
4. Read the results: You get the raw volume plus the bag count for 40 lb, 60 lb and 80 lb pre-mixed bags.

The Formulas Behind It

• Slab or footing: length × width × depth = volume. Keep every measurement in the same unit before multiplying.
• Column or cylinder: π × radius² × height. Remember the radius is half the diameter — using the full diameter here is the single most common error, and it produces four times too much concrete.
• Cubic feet to cubic yards: divide by 27, because a cubic yard is 3 × 3 × 3 feet.
• Bag yields: an 80 lb bag makes roughly 0.60 cu ft, a 60 lb bag roughly 0.45 cu ft, and a 40 lb bag roughly 0.30 cu ft of mixed concrete.

Typical Thicknesses

Depth matters more than people expect — going from 4 inches to 6 inches adds 50% to your material cost. These are common starting points, but always follow your local building code and any structural engineer's specification.

Ordering: Bags or Ready-Mix?

The crossover point is usually somewhere around a cubic yard, or about half a cubic metre.

• Bags make sense for small jobs — posts, small pads, repairs. They're easy to buy, store and mix in batches. Beyond about 40–50 bags, mixing becomes a genuinely exhausting day's work and it gets hard to keep the pour continuous.
• Ready-mix delivery is cheaper per unit volume, arrives consistently mixed, and lets you place a large slab in one continuous pour. Suppliers usually have a minimum order and may charge for part loads, so check before ordering.

Always Add a Waste Allowance

This calculator gives you the exact theoretical volume. Real sites are not exact: sub-grades are uneven, forms bow slightly under pressure, and some concrete is always lost to spillage and to the mixer. Add 5–10% for a well-prepared slab and closer to 10–15% for footings dug into soft or irregular ground, where the trench is invariably wider than you planned.

Practical Tips Before You Pour

• Prepare the sub-base. Compacted hardcore or gravel under a slab prevents settlement cracks later. Concrete is strong in compression but unforgiving of movement beneath it.
• Check your forms. Wet concrete is heavy and pushes hard against formwork — brace it properly, or a bowed edge will be set in stone.
• Plan for control joints. Concrete shrinks slightly as it cures and will crack. Cutting or tooling joints decides where those cracks happen. A common guideline is joints spaced at 24–30 times the slab thickness.
• Mind the weather. Very hot conditions make concrete set too quickly to finish properly; near-freezing temperatures stop it curing altogether. Both weaken the finished result.
• Cure it, don't just leave it. Keeping concrete damp for several days after pouring lets it reach a substantially higher final strength than letting it dry out.
• Have enough hands. Once concrete arrives, the clock is running. Placing and finishing a large slab is not a one-person job.