Concrete & masonry · free tool
Concrete Calculator
How much concrete do you need? Enter your slab, footing, column, or post hole and get the exact cubic yards, the 40/60/80-lb bags to buy, ready-mix, and cost — with waste built in.
Concrete Calculator
How much concrete you need — yards, bags & cost.
1 Your pour
2 Your estimate
Bags are cheaper at this size.
A 80-lb bag makes ~0.60 ft³ · about 47 gal water for 62 bags.
Plants set a ~1 yd³ minimum; short-load fees apply under ~5 yd³.
Estimates only — bag yield and local density vary; order the waste shown. Checked against Quikrete/Sakrete yields. See how we calculate · Updated monthly
Method · how it works
How to calculate how much concrete you need
The math is simple — here is exactly what the calculator does, so you can check it by hand.
Find the volume
Multiply length × width × thickness in feet, then divide by 27. Round columns use π·r²·h; a post hole is the hole minus the post.
Convert to bags
Divide cubic feet by the bag yield. An 80-lb bag makes 0.60 ft³, 60-lb 0.45, 40-lb 0.30 — so a yard is 45, 60, or 90 bags.
Add waste
Order 5–10% extra for spillage, uneven subgrade and over-dig, so you never come up short mid-pour.
Or order ready-mix
Above about 1–2 yd³ a delivered truck is usually cheaper and faster than bags. Round up to the nearest 0.25 yard.
A 10 × 10 ft slab, 4 in thick
33.3 ÷ 27 = 1.23 yd³
33.3 ÷ 0.60 × 1.10 = 62 bags
62 × 80-lb bags (10% waste)
or order 1.5 yd³ ready-mix
Sources: Quikrete · Sakrete · ACI bag-yield & density references
Reviewed against manufacturer specs · Updated monthly
Estimates only. Bag yield, compaction, and local material density vary — check the printed yield on your bag and order the waste margin shown.
Slab depth · by project
How thick should a concrete slab be?
Thickness follows the load. Here are the standard depths — pick your project in the calculator above and it sets the thickness for you.
Light foot traffic and outdoor living.
Wire meshWalkways and garden paths.
Wire meshGo 5 in for heavy storage or equipment.
Wire meshCars and light trucks.
Rebar 12–18 in o.c.Slab-on-grade floors.
Rebar + vapor barrierStructural and heavy loads.
RebarLocal codes and soil conditions can call for more — see how we calculate.
The money decision
Bagged concrete vs ready-mix: which to buy?
The cheaper choice flips with the size of your pour. Here is how they compare — and where the switch happens.
Best for: post holes, footings, small pads, repairs
Best for: slabs, driveways, garages
The switch point
Around 1–2 yd³, ready-mix overtakes bags.
The calculator above shows this live for your pour.
Quick reference
How much concrete for common slab sizes
Exact cubic yards and 80-lb bags for the sizes people ask about most, at the two most common depths.
| Slab size | At 4 in | At 6 in |
|---|---|---|
| 10 × 10 ft | 1.23 yd³ 56 × 80-lb bags | 1.85 yd³ 83 bags |
| 12 × 12 ft | 1.78 yd³ 80 × 80-lb bags | 2.67 yd³ 120 bags |
| 16 × 16 ft | 3.16 yd³ 142 × 80-lb bags | 4.74 yd³ 213 bags |
| 20 × 20 ft | 4.94 yd³ 222 × 80-lb bags | 7.41 yd³ 333 bags |
| 20 × 24 ft | 5.93 yd³ 267 × 80-lb bags | 8.89 yd³ 400 bags |
| 24 × 24 ft | 7.11 yd³ 320 × 80-lb bags | 10.67 yd³ 480 bags |
Exact volumes — add 5–10% for waste. Need a different size, depth, or ready-mix and cost? Change it in the calculator.
Mixing it right
Concrete mix ratio, water & strength
If you are mixing bags yourself, the ratio and the water set the strength. Here are the standard mixes — and the one mistake to avoid.
| For | Mix · cement : sand : stone | Strength |
|---|---|---|
| Generalpaths, patios | 1 : 2 : 4 | 2,500–3,000 PSI |
| Standardslabs, driveways | 1 : 2 : 3 | 3,000–3,500 PSI |
| Structuralfootings, columns | 1 : 1.5 : 3 | 4,000–5,000 PSI |
Water per bag
Start dry, add water gradually to a stiff, workable mix.
Extra water weakens concrete. Just one extra quart in an 80-lb bag can drop it from 4,000 to ~2,500 PSI — about 300 PSI lost per 10 lb of water. Mix it stiff, not soupy.
Curing & timing
How long does concrete take to cure?
Concrete keeps hardening for weeks. Here is when it is ready for each kind of use.
Poured
Keep it damp as it sets
24–48 hrs
Walk on it
7 days
Drive & build on it (~70%)
28 days
Full strength
Pour between 50–80°F. Below 40°F concrete barely gains strength; in heat, pour early morning and avoid the hottest hours (11am–3pm). Keeping it damp for the first few days makes it stronger.
Do it right
Rebar, base & prep
A strong slab is mostly what happens before the pour. The essentials:
Reinforcement
Before you pour
Answers
Concrete calculator FAQs
How many bags of concrete are in a yard?
A cubic yard takes about 45 × 80-lb bags, 60 × 60-lb, or 90 × 40-lb bags — because a yard is 27 cubic feet and an 80-lb bag makes 0.60 ft³.
How much does a yard of concrete weigh?
About 4,050 lb (roughly 2 tons). Concrete weighs around 150 lb per cubic foot, and a cubic yard is 27 cubic feet.
How do I calculate concrete for a slab?
Multiply length × width × thickness in feet, then divide by 27 for cubic yards: yd³ = L × W × D ÷ 27. A 10 × 10 ft slab at 4 in is 1.23 yd³. Use the calculator to skip the math.
How thick should a concrete slab be?
4 in for patios, sidewalks, and sheds; 5–6 in for driveways with rebar; 6 in for garage floors. Structural footings go 6 in or more.
Should I buy bags or order ready-mix?
Under about 1 yd³, bags are cheaper and simpler. Over about 2 yd³, ready-mix delivery usually wins on price and time. In between, compare your bag price against a short-load delivery.
How much water goes in an 80-lb bag?
About 3 quarts (0.75 gal). Start dry and add gradually — too much water sharply weakens the mix.
How long before I can walk or drive on it?
Walk on it after 24–48 hours, drive or build on it after 7 days (~70% strength), and it reaches full strength at 28 days.
How much extra concrete should I order?
Add 5–10% for spillage, uneven subgrade, and over-dig, so you never run short mid-pour. The calculator builds this in.
What is the strongest concrete mix?
A 1 : 1.5 : 3 (cement : sand : stone) mix reaches about 4,000–5,000 PSI for structural work. A standard 1 : 2 : 3 slab mix is 3,000–3,500 PSI.
Is this concrete calculator free?
Yes — it is completely free, with no sign-up. It gives cubic yards, bags by size, ready-mix, weight, and cost for any slab, footing, column, or post hole.