How to Calculate Concrete

Concrete Volume Formulas for Every Pour Type

Use this formula sheet as a quick reference before you move into the detailed examples. Every formula below returns cubic feet when the dimensions are entered in feet. Divide by 27 after the formula if you need cubic yards for ordering.

How to Calculate Concrete for a Circular Slab

Circular slabs appear around fire pits, tank pads, and decorative patios. The only difference from a rectangular slab is the plan shape. Instead of multiplying length by width, you use pi x radius squared to find the area, then multiply by thickness.

Volume (ft3) = pi x (D / 2)^2 x T
Volume (yd3) = pi x r^2 x T / 27

D = diameter in feet
r = radius = D / 2
T = thickness in feet

Step 1 - Convert thickness to feet:
4 in / 12 = 0.333 ft

Step 2 - Calculate radius:
r = 10 / 2 = 5 ft

Step 3 - Volume in cubic feet:
pi x 5^2 x 0.333 = 26.18 ft3

Step 4 - Convert to cubic yards:
26.18 / 27 = 0.97 yd3

Step 5 - Add overage and convert to bags:
0.97 x 1.1 = 1.07 yd3 = about 49 bags

The most common error on round work is using the diameter where the formula needs radius. Always divide the diameter by two first. A second mistake is converting diameter to feet but forgetting to convert thickness from inches to feet. If both conversions happen before the formula, the rest of the math is routine.

How to Calculate Concrete for a Pad Footing

Pad footings are isolated blocks under columns, deck piers, and equipment bases. Because the geometry is still rectangular, the formula is identical to the slab and strip footing formula. The only added step is multiplying by quantity when the project uses several identical pads.

Volume (ft3) = Length x Width x Depth
Volume (yd3) = L x W x D / 27

For multiple identical pads:
Total Volume = Single Pad Volume x Count

Step 1 - Convert to feet:
L = 24 / 12 = 2.0 ft
W = 24 / 12 = 2.0 ft
D = 12 / 12 = 1.0 ft

Step 2 - Single pad volume:
2.0 x 2.0 x 1.0 = 4.0 ft3
4.0 / 27 = 0.15 yd3

Step 3 - Total for 6 pads:
0.15 x 6 = 0.89 yd3

Step 4 - Add overage and convert to bags:
0.89 x 1.1 = 0.98 yd3 = about 45 bags

Pad footings are a good example of why quantity belongs in the calculation and not in your head. A single small footing might feel trivial, but six or eight identical pads can quickly push the project close to ready-mix territory. If the pads are repeated, calculate one correctly, then multiply by the count before adding overage.

How to Calculate Concrete for a Round Column or Tube Form

Round columns, Sonotubes, and post piers use cylinder geometry. That means pi x radius squared x height. This is one of the most searched residential concrete calculations because deck piers and fence posts are frequently built with cardboard tube forms or round drilled holes.

Volume (ft3) = pi x (D / 2)^2 x H
Volume (yd3) = pi x r^2 x H / 27

D = diameter in feet
H = height or fill depth in feet

Step 1 - Convert diameter to feet:
12 / 12 = 1.0 ft, so radius = 0.5 ft

Step 2 - Single tube volume:
pi x 0.5^2 x 4 = 3.14 ft3
3.14 / 27 = 0.12 yd3

Step 3 - Total for 6 tubes:
0.12 x 6 = 0.70 yd3

Step 4 - Add overage and convert to bags:
0.70 x 1.1 = 0.77 yd3 = about 35 bags

Tube-form calculations are easy to double-check because the same diameter and height tend to repeat across the whole project. That repetition makes them ideal for one verified per-tube number multiplied by the total quantity. It is also why many people remember the 12-inch by 4-foot deck-pier rule of thumb: it lands at roughly six 80 lb bags per tube with overage included.

How to Calculate Concrete for Stairs

Concrete stairs are different from slabs, walls, and footings because the shape is a stack of repeated step blocks. Treating the stair as one simple rectangle overestimates the volume badly. The standard solid-stair formula accounts for the stepped geometry by multiplying one step block by N(N+1)/2.

Volume (ft3) = W x Rise x Run x N x (N + 1) / 2

W = stair width in feet
Rise = step rise in feet
Run = tread depth in feet
N = number of steps

Step 1 - Convert rise and run to feet:
Rise = 7 / 12 = 0.583 ft
Run = 11 / 12 = 0.917 ft

Step 2 - Apply the stair formula:
V = 4 x 0.583 x 0.917 x 5 x 6 / 2
V = 32.1 ft3

Step 3 - Convert to cubic yards and add overage:
32.1 / 27 = 1.19 yd3
1.19 x 1.1 = 1.31 yd3

Step 4 - Bag count:
1.31 yd3 = about 60 bags of 80 lb mix

Stair math is where a dedicated calculator earns its keep, especially when the stair width stays constant but the rise or run changes from standard proportions. Even when you do the arithmetic by hand, the best workflow is still to convert rise and run to feet first and keep the formula organized line by line. That avoids the unit mistakes that make stair takeoffs frustrating.

Unit Conversion Quick Reference

These are the unit conversions you will reach for most often while calculating concrete. The first table handles thickness and diameter conversions. The second table is the cubic-foot to cubic-yard check that shows up in almost every order calculation.

Frequently Asked Questions

These FAQ answers cover the high-frequency searches around cubic yards, slab formulas, round columns, irregular pours, and when it makes sense to switch from bagged concrete to ready-mix delivery.