How to Calculate How Much Concrete You Need for Any Project
Here's a situation almost every DIYer knows: you're standing in the concrete aisle, staring at a stack of bags, mentally calculating how many you need — and silently terrified you'll either run short halfway through or end up hauling eight unused bags back to the car.
Getting concrete quantities wrong is one of the most frustrating (and heavy) mistakes in home improvement. The good news is the maths involved is genuinely simple. Whether you're setting fence posts, pouring a shed slab, or laying footings for a deck, this guide will walk you through exactly how to calculate what you need — in both metric and imperial — before you buy a single bag.
Understanding Concrete Volume: The Basics
Concrete is always sold or quoted by volume:
- Imperial: cubic feet (cu ft) or cubic yards (cu yd)
- Metric: cubic metres (m³) or litres (L)
The goal is always the same: calculate the volume of the space you need to fill, then convert that to bags or ready-mix quantities.
Key conversion facts to keep handy:
- 1 cubic yard = 27 cubic feet
- 1 cubic metre = 1,000 litres = 35.3 cubic feet
- 1 cubic foot ≈ 28.3 litres
If you need to convert between units, our Volume Converter handles all of these in one step.
Project Type 1: Post Holes (Fence Posts, Deck Posts, Letterbox Posts)
Post holes are the most common reason people buy bags of concrete — and the easiest to calculate. A post hole is essentially a cylinder (minus the post itself, but for bag estimation, ignoring the post gives you a useful safety margin).
The volume of a cylinder is:
Where π ≈ 3.14159, radius = half the hole diameter, and depth = how deep the hole is.
Imperial Example
You're setting fence posts with holes that are 10 inches wide and 30 inches deep.
- Radius = 10 ÷ 2 = 5 inches
- Volume = 3.14159 × 5² × 30 = 2,356 cubic inches
- Convert: 2,356 ÷ 1,728 = 1.36 cubic feet per hole
With 10 post holes: 13.6 cubic feet total (before the 10% waste buffer — more on that below).
Metric Example
You're setting posts in holes that are 250 mm wide (25 cm) and 750 mm deep (75 cm).
- Radius = 0.125 m
- Volume = 3.14159 × 0.125² × 0.75 = 0.037 m³ per hole (37 litres)
With 10 post holes: 0.37 m³ total (370 litres).
Rather than working through this manually, use our Cylinder Calculator — enter the radius and depth and it calculates the volume immediately. If you're planning a full fence, our Fence Calculator can estimate your post count and material quantities all at once.
Project Type 2: Concrete Slabs (Patios, Shed Bases, Driveway Aprons)
Slabs are rectangular (or square) volumes — the formula is simply:
The tricky part is keeping your units consistent. Convert everything to the same unit before multiplying.
Imperial Example
A patio slab: 12 ft long × 10 ft wide × 4 inches thick.
- Convert thickness: 4 inches = 4 ÷ 12 = 0.333 ft
- Volume = 12 × 10 × 0.333 = 40 cubic feet
- In cubic yards: 40 ÷ 27 = 1.48 cubic yards
At this volume, ordering ready-mix concrete from a supplier becomes cost-effective compared to buying bags. Most suppliers deliver from around 1 cubic yard (0.75 m³) minimum.
Metric Example
A shed base: 3 m long × 2.5 m wide × 100 mm thick.
- Convert thickness: 100 mm = 0.1 m
- Volume = 3 × 2.5 × 0.1 = 0.75 m³ (750 litres)
Use our Concrete Calculator to handle slab calculations automatically — just enter your dimensions and it outputs both volume and bag counts.
Project Type 3: Footings and Strip Foundations
Footings for decks, garden walls, or small structures are typically rectangular trenches — the same formula as slabs:
For an L-shaped or complex footing layout, break it into rectangular sections, calculate each separately, then add them together.
Imperial Example
A strip footing: 20 ft long × 1 ft wide × 1 ft deep.
- Volume = 20 × 1 × 1 = 20 cubic feet = 0.74 cubic yards
Metric Example
A footing: 6 m long × 300 mm wide × 300 mm deep.
- Volume = 6 × 0.3 × 0.3 = 0.54 m³ (540 litres)
Again, our Concrete Calculator is the fastest way to get from measurements to bag count.
Converting Volume to Bags: Imperial and Metric
⚠️ Check the Bag Label
The yields below are industry averages for standard general-purpose mix. Fast-setting concrete (e.g. Quikrete Fast-Setting, Postcrete) can yield slightly differently, and some premium mixes are denser. Always check the yield printed on the specific bag you're buying before finalising your order.
Once you have your total volume, you need to translate it into bags to buy. Bag yield varies by size and brand, but these are the standard figures:
Imperial Bags
| Bag Size | Approximate Yield |
|---|---|
| 40 lb bag | ~0.30 cu ft |
| 60 lb bag | ~0.45 cu ft |
| 80 lb bag | ~0.60 cu ft |
Formula: Number of bags = Total cubic feet ÷ Yield per bag
For our 40 cu ft patio: 40 ÷ 0.60 = 67 bags of 80 lb (round up to 74 after adding 10% waste).
Metric Bags
| Bag Size | Approximate Yield |
|---|---|
| 20 kg bag | ~10 litres (0.010 m³) |
| 25 kg bag | ~12.5 litres (0.0125 m³) |
| 30 kg bag | ~15 litres (0.015 m³) |
Formula: Number of bags = Total litres ÷ Yield per bag
For our 0.75 m³ shed base (750 litres): 750 ÷ 12.5 = 60 bags of 25 kg (round up to 66 after adding 10%).
The 10% Waste Rule: Always Buy More Than You Think You Need
Industry standard rule: Always add 10% to your calculated concrete volume before purchasing. No hole is perfectly cylindrical, no form is perfectly rigid, and soil always compresses slightly — the 10% buffer ensures you don't run short mid-pour.
No hole is perfectly cylindrical. No slab is perfectly flat. Soil compresses, soil crumbles, and forms flex. In practice, you'll always use slightly more concrete than the theoretical calculation suggests.
The industry standard is to add 10% to your calculated volume before purchasing.
If your calculation says you need 40 cubic feet, buy for 44 cubic feet. If you need 60 bags, buy 66 bags.
Our Percentage Calculator can instantly add 10% (or any other margin) to any number you're working with.
💡 Pro Tip: When to Order Ready-Mix Instead
Mixing bags by hand is fine for small jobs — a few post holes or a small pad. But once you're above roughly 1 cubic yard (0.75 m³), the time and effort of mixing dozens of heavy bags often makes ordering ready-mix concrete from a local supplier the smarter choice. Ready-mix is priced per cubic metre or yard, typically requires a minimum order, and is delivered wet and ready to pour. Call your local supplier for pricing — for anything over 1 m³, it's almost always worth the comparison.
Key Takeaway
Every concrete job is a volume calculation — and every volume is just a simple shape. Post holes are cylinders (π × r² × depth). Slabs and footings are rectangles (length × width × thickness). Calculate in consistent units, convert to bags using standard yields, and always add 10% for waste. You'll never over- or under-buy again.
⚠️ Mind the Temperature
Don't pour concrete when temperatures are forecast to drop below 4 °C (40 °F) within 24 hours of the pour, or when it's above 32 °C (90 °F) at time of mixing. Cold weather slows cure and risks frost damage to the set concrete; extreme heat causes it to cure too fast and crack. Most pre-mix bags include temperature guidance on the label.
Frequently Asked Questions
How long does bagged concrete take to set?
Initial set (firm enough to leave alone) typically takes 24–48 hours. Full cure takes 28 days, though concrete reaches most of its working strength within the first week. Temperature significantly affects curing time — cold weather slows it, hot dry weather can cause it to cure too fast and crack. In cold climates, avoid pouring when temperatures are expected to drop below 4°C (40°F) without frost protection measures.
Should I put gravel at the bottom of post holes?
Yes, and it's highly recommended if you're using timber posts. A 100–150 mm (4–6 inch) layer of gravel or crushed rock at the base of each hole provides drainage, which significantly extends the life of wooden posts by preventing moisture from sitting at the base. Remember to subtract your gravel depth from the hole depth when calculating how much concrete you need.
What concrete mix is right for fence posts vs. slabs?
For fence and deck posts, a standard fast-setting or general-purpose mix (like N20 in Australia, C20 in the UK, or standard 60/80 lb bags in the US) is typically sufficient. For structural slabs and footings, a stronger mix is usually recommended — check with your local building authority for any required specifications if your project needs a permit. Your hardware store or concrete supplier can also advise on the right mix for your climate and soil conditions.
What's the difference between concrete, mortar, and cement?
Cement is an ingredient — the binding powder. Concrete is cement + sand + aggregate (gravel or crushed rock) + water. Mortar is cement + sand + water (no aggregate), used for laying bricks and blocks. The bagged products at hardware stores labelled "concrete mix" include all the dry ingredients — you just add water. Never substitute mortar for concrete in structural applications like post footings.
What if my post hole isn't perfectly round?
Don't stress about it. If the hole is slightly oval, measure the widest and narrowest diameter, add them together, and divide by two to get an average diameter — then use half of that as your radius. This approximation is close enough for calculating bag quantities, and the 10% waste buffer covers any remaining uncertainty.
Calculate Your Concrete Now
Ready to get an accurate number before you head to the store? Our Concrete Calculator walks you through post holes, slabs, and footings — with results in both metric and imperial. No guessing, no overbuying, no wasted trips.
