Concrete Calculator: Bags, Cubic Yards & Waste Factor

Concrete Calculator: Estimate Bags & Cubic Yards for Your Project

Free concrete calculator for slabs, footings, columns, stairs, and curbs. Get bag counts and cubic yards with a waste factor built in. Imperial and metric.

How Much Concrete Do I Really Need?

Why the Math Isn't Enough

Uneven ground:

Low spots and rough subgrade eat extra concrete to keep thickness consistent.
Spillage and over-excavation:

You lose some in the chute and sometimes overfill forms to get full coverage.
What to do:

Add a buffer—we let you set 0–20%, default 10%—so you order enough without guessing.

Ordering exactly what the formula gives you often leads to running short. Subgrade isn't perfectly level, concrete spills, and forms get overfilled. Contractors add a waste factor to cover it.

How Much Waste to Add

Simple slabs (5%):

Level site, straightforward forms. Patios, small driveways.
Most residential (10%):

Our default. Covers typical subgrade and spillage.
Stairs, columns, tight spots (15–20%):

More formwork, more waste. Bump it up so you don't run out.

It depends on the job.

Concrete Volume by Project Type

Slabs and Footings

Formula:

V = L × W × T
where V is volume, L is length, W is width, and T is thickness (all in same units).
Example:

10 ft × 10 ft at 4 in thick: 10 × 10 × (4/12) = 33.33 cubic feet ≈ 1.23 cubic yards.
Rough thickness:

Driveways 4–6 in. Patios and sidewalks 4 in. Foundations 6–8 in.

Length × width × thickness. Same units for all three.

Round Columns

Formula:

V = π × r² × H
where V is volume, r is radius (diameter ÷ 2), and H is height/depth.
Example:

12-in diameter, 8 ft tall: π × (0.5)² × 8 = 6.28 cubic feet ≈ 0.23 cubic yards.
Multiple columns: do one, multiply by count, then add waste to the total.

Cylinder volume: π × radius² × height. Radius = diameter ÷ 2.

Stairs

Formula:

V_step = R_u × R_i × W
V_total = V_step × N
where R_u is run (horizontal depth), R_i is rise (height), W is width, N is number of steps.
Example:

5 steps, 10" run × 7" rise × 36" width: (10/12) × (7/12) × (36/12) × 5 ≈ 4.86 cubic feet ≈ 0.18 cubic yards.
Stairs eat more waste—formwork and tight spaces. Use 10–15% if you're doing them by hand.

Volume per step = run × rise × width. Add the steps (or use the Stairs shape above).

Curb and Gutter

Formula:

V_curb = L × H × D
V_gutter = L × W × D_g
V_total = V_curb + V_gutter
where L is length, H curb height, D curb depth, W gutter width, D_g gutter depth.
Example:

50 ft: curb 6" × 6" = 12.5 cu ft; gutter 12" × 4" = 16.67 cu ft. Total ≈ 29.2 cu ft ≈ 1.08 cubic yards.
Residential curb often 6" × 6"; gutter 12" × 4". Commercial can be bigger.

Curb volume plus gutter volume (both are length × cross-section). Add the flag if you have one.

Bagged vs. Ready-Mix

Bagged: 40, 60, 80 lb

40 lb:

0.011 cubic yards (0.30 cu ft) per bag. Small repairs, under half a yard.
60 lb:

0.017 cubic yards (0.45 cu ft) per bag. Middle ground.
80 lb:

0.022 cubic yards (0.60 cu ft) per bag. Fewer bags per yard.
We show counts for all three once you have your volume.

Different bag sizes, different yields. Divide your cubic yards by the yield per bag and round up.

Ready-Mix by Truck

Minimum:

Most suppliers want at least 1 cubic yard. Under that, bags are the way to go.
Ordering:

Order in 0.25 or 0.5 yard increments. Round up after you've added waste.
Check truck access and pour timing—you need a continuous pour once the truck arrives.

Past about a yard, ready-mix usually beats bags on price and speed. No mixing—just place it.

Weight and Transport

How Heavy Is It?

Per cubic yard:

~4,000 lbs (1,814 kg).
Per cubic meter:

~2,400 kg (5,291 lbs).
Typical trucks carry 8–10 cubic yards. Confirm access and that forms and subgrade can support the pour.

Wet concrete runs about 4,000 lbs per cubic yard (2,400 kg/m³). That drives truck choice and whether your base can take the load.

FAQ

How much concrete do I really need?

Ordering exactly what the math says often leads to running short—subgrade irregularities, spillage, and uneven forms eat into your supply. Pros add 5–10% waste; we default to 10% and you can adjust so you finish the pour without over-ordering.

How thick should a driveway slab be?

Residential driveways: 4–6 inches. Heavier traffic or commercial use: 6–8 inches. Enter your length, width, and thickness here and we'll add waste so you get one number to order.

How many 80lb bags are in a cubic yard?

Roughly 45 bags of 80lb mix per cubic yard—each bag yields about 0.022 cubic yards (0.60 cubic feet). We show bag counts for 40lb, 60lb, and 80lb once you enter your volume.

What is the difference between bagged and ready-mix concrete?

Bagged comes in 40lb, 60lb, or 80lb bags—handy for small jobs. Ready-mix is delivered by truck with a 1-yard minimum. Past a yard, ready-mix usually wins on cost. We give you both bag counts and cubic yards so you can compare.

How do I calculate concrete for stairs?

Each step: run × rise × width. Example: 10" deep × 7" high × 36" wide ≈ 0.97 cubic feet per step. Multiply by the number of steps. Use the Stairs option above and we'll do it.

What is a waste factor and why do I need it?

Waste covers spillage, over-excavation, uneven forms, and rough subgrades. Pros add 5–10%. Need 10 cubic yards? Order 11 with 10% waste. Running out mid-pour means weak cold joints and rework.

How much does concrete weigh?

Wet concrete runs about 4,000 lbs per cubic yard (2,400 kg/m³). That affects truck capacity, formwork, and whether your base can support the load. We multiply your volume by that so you see estimated weight.

What is the minimum order for ready-mix concrete?

Most suppliers have a 1 cubic yard minimum. Under a yard, bags usually make more sense. We show both so you can pick the cheaper route.

Concrete Calculator

Project dimensions

Concrete Calculator: Bags, Yards, and Why the Math Isn't Enough

What Actually Eats Your Concrete

Why Pros Add Waste

Bags vs. Truck

Weight Matters

Slabs + Columns + Stairs