Concrete is unforgiving about quantity: too little and you stop mid-pour with a weak cold joint, too much and you have paid to dispose of the extra. This calculator converts a slab’s length, width, and thickness into cubic feet and cubic yards, then into the number of pre-mix bags — 80 lb or 60 lb — if you are mixing it yourself.
It suits the standard home pours: patio and shed slabs, walkways, driveway extensions, and post footings laid out as rectangles. Enter thickness in inches, the way slabs are actually specified.
Concrete Calculator
Worked example: a 12 × 10 ft patio slab, 4 in thick
- Volume: 12 × 10 × (4 ÷ 12) = 40 cubic feet
- In cubic yards: 40 ÷ 27 = 1.48 cu yd — about 1.5 cu yd to order as ready-mix
- In 80 lb bags: 40 ÷ 0.6 = 66.7, rounded up to 67 bags
- In 60 lb bags: 40 ÷ 0.45 = 88.9, rounded up to 89 bags
That bag count is why the bags-versus-ready-mix decision matters: 67 bags is roughly a yard and a half of concrete mixed one bag at a time. For a pour this size, most people find a ready-mix truck — or at least a rented drum mixer — the better day.
How the calculation works
Volume = length × width × thickness, with thickness converted to feet (inches ÷ 12). One cubic yard is 27 cubic feet.
Bag counts use the yields printed by major US pre-mix manufacturers: one 80 lb bag yields about 0.6 cu ft of mixed concrete, and one 60 lb bag yields about 0.45 cu ft. Bags = volume ÷ yield, always rounded up, because a slab that is 95% poured is not 95% finished. The calculator reports the exact volume for ready-mix; when you order, expect the supplier to round to their nearest increment and to suggest a small overage for uneven subgrade, which is normal practice.
Typical concrete costs
Typical US prices, checked November 2026. Materials only; ready-mix delivery and short-load fees are extra and vary by supplier and region.
| Material | Typical cost |
|---|---|
| 80 lb pre-mix bag | $5.00 – $7.00 per bag |
| 60 lb pre-mix bag | $4.50 – $6.50 per bag |
| Ready-mix concrete | $130 – $170 per cu yd, plus delivery |
For the worked example: 67 bags at retail comes to roughly $335 – $470, while 1.5 cu yd of ready-mix is about $200 – $255 before delivery fees — the reason larger pours usually go to the truck.
Before you pour
- Measure the formed area, not the planned area — forms move, and the slab is the size of the hole you actually built.
- Confirm the thickness your project and local rules call for before ordering; thickness changes the volume fast (going from 4 in to 5 in adds 25%).
- Decide bags versus ready-mix by volume: under about half a cubic yard, bags are manageable; over a cubic yard, price the truck.
- Prepare the base first — a compacted gravel layer under the slab is standard, and the Gravel Calculator estimates that material in tons.
- Line up help, tools, and the water supply before any concrete arrives or the first bag is opened. Concrete sets on its own schedule.
- If the slab sits on a yard you are also landscaping, plan volumes together — the Mulch Calculator covers the beds that usually surround a new patio.
FAQ
How many bags of concrete do I need for a 10 × 10 ft slab?
At 4 in thick, the slab is 33.3 cu ft. That is 56 bags of 80 lb mix or 75 bags of 60 lb mix — or about 1.23 cu yd of ready-mix.
How many bags of concrete make a cubic yard?
About 45 bags of 80 lb mix or 60 bags of 60 lb mix. It is a useful reality check: anything over a yard is a lot of individual bags to lift, mix, and place.
How thick should a patio slab be?
Four inches is the common thickness for patios and walkways that carry foot traffic only. Anything that will carry vehicles, or ground that freezes hard, may need more, and local codes can set minimums — confirm before you order rather than copying a neighbor’s slab.
Should I order a little extra ready-mix?
Yes, a small margin — typically 5–10% — covers subgrade that is slightly uneven and the concrete that stays in the chute. The calculator gives the exact geometric volume; add the margin when you talk to the supplier, not by inflating your measurements.
How much does a cubic yard of concrete weigh?
Roughly 4,000 lb (about 2 tons) once mixed. That weight is why delivery method matters: the 67 bags in the worked example total over 5,300 lb of dry mix before water.
Related calculators
The Square Footage Calculator confirms the footprint area that starts this estimate. Under the slab, the Gravel Calculator works out the compacted base in tons and bags, and for the classic alternative to a concrete patio, the Deck Board Calculator counts the boards for a wood or composite deck of the same size.
Estimates only: results are based on the standard volume formula and manufacturers’ published bag yields, and actual quantities vary with subgrade, form accuracy, and waste. Confirm your measurements before ordering. These tools do not cover structural, load-bearing, electrical, gas, or plumbing design, and they do not size footings or reinforcement — for safety-critical work, hire a licensed professional.