Cubic Feet Calculator for Concrete: How to Measure

A cubic feet calculator for concrete works out the volume of concrete you need for a pour, using the length, width, and depth (or height) of the form. You enter those three dimensions in feet (or inches and feet), and the calculator returns the volume in cubic feet, which you can then convert to cubic yards for ordering. This page gives you the calculator, the formula behind it, a step-by-step example, and the common pitfalls that lead to under- or over-ordering.

Formula

The basic formula for concrete volume is the same as for any rectangular solid:

Volume (ft³) = Length (ft) × Width (ft) × Depth (ft)

Because depth is usually given in inches on plans, you first convert it to feet by dividing by 12. The calculator uses this expression directly:

V = L × W × (D / 12)

Where:

• V = concrete volume in cubic feet (ft³)
• L = length of the pour in feet (ft)
• W = width of the pour in feet (ft)
• D = depth or thickness in inches (in), divided by 12 to get feet

For a round column or footing, the formula becomes V = π × r² × h, where r is the radius in feet and h is the height in feet. For a triangular-shaped slab (like a stair landing), it is V = (base × height / 2) × depth.

Worked Example

Using the default values from the calculator above — a slab that is 10 feet long, 4 feet wide, and 4 inches deep — here is the step-by-step calculation:

1. Convert depth to feet: 4 in ÷ 12 = 0.3333 ft
2. Multiply length × width × depth: 10 × 4 × 0.3333 = 13.33 ft³
3. Convert to cubic yards (for ordering): 13.33 ÷ 27 = 0.49 yd³
4. Estimate 60-lb bags: 13.33 ÷ 0.45 ≈ 29.6, so round up to 30 bags

The calculator shows 13.33 cubic feet, 0.49 cubic yards, and 30 bags. That matches the manual math.

When to Use Cubic Feet vs. Cubic Yards

Concrete is sold by the cubic yard in most regions, but the volume of a small pour — a sidewalk, a shed base, a single footing — is often easier to visualize in cubic feet. If you are ordering ready-mix, you will convert cubic feet to cubic yards by dividing by 27 (since 1 yard = 3 feet, and 3³ = 27). If you are mixing bags, each 60-lb bag yields roughly 0.45 cubic feet (a 80-lb bag yields about 0.6 ft³).

Use the cubic feet number when you are comparing bag quantities or when you are working with a form that is measured in feet and inches. Use cubic yards when you call a ready-mix supplier, because that is the unit they quote and bill.

How to Measure a Concrete Pour Correctly

Getting accurate measurements is the difference between a neat job and a short pour. Follow these steps:

• Measure the inside of the form, not the outside. The form boards are the true boundary of the concrete.
• For a slab, measure length and width at the widest points, and take depth at several spots — ground is rarely perfectly level. Use the average depth.
• For a footing or beam, measure the actual cross-section dimensions, not the nominal lumber size (a 2×6 is actually 1.5 in × 5.5 in).
• For a round column, measure the diameter of the inside of the tube, then divide by 2 to get the radius.
• Always measure in the same unit. If you mix feet and inches, convert everything to feet first.

Common Mistakes and How to Avoid Them

The most frequent error is forgetting to convert inches to feet. A 10 ft × 4 ft slab at 4 inches deep is 13.33 ft³, but if you treat 4 inches as 4 feet, you get 160 ft³ — a huge overorder. Another common mistake is using the outside dimensions of a form when the inside is smaller, which adds waste.

Rounding errors also creep in. If you round 0.3333 ft to 0.3 ft, you lose about 10% of the volume. Keep at least two decimal places during calculation, then round the final number up when ordering.

Finally, do not forget that concrete shrinks slightly as it cures, but that shrinkage is negligible for ordering purposes. What matters is that you order a little extra (5–10%) to account for spillage, uneven ground, or slight measurement errors. The calculator’s bag estimate already rounds up, but for ready-mix, add 10% to the cubic yard figure.

Cost and Ordering Considerations

While this calculator only computes volume, the cost of concrete depends on that volume. Ready-mix is typically quoted per cubic yard, and most suppliers have a minimum order (often 1 yard). If your pour is less than that, you will pay a short-load fee, so it may be cheaper to mix bags. For example, a 0.49-yard pour is less than the minimum, so buying 30 bags of 60-lb mix might be more practical than ordering a truck.

Bagged concrete is sold in 40, 60, or 80-pound bags. A 40-lb bag makes about 0.30 ft³, a 60-lb bag makes 0.45 ft³, and an 80-lb bag makes 0.60 ft³. Divide your cubic feet by those numbers and round up to the nearest whole bag.

When ordering ready-mix, you will need to know the volume in cubic yards, and you should also ask about delivery fees and whether the driver can reach the pour site with a chute. Those factors are outside the scope of this calculator, but they affect the final cost.

Alternatives and Special Shapes

Not every pour is a simple rectangle. For an L-shaped slab, break it into two rectangles, calculate each, and add them together. For a circular patio, use the formula π × r² × depth. For a trapezoidal footing, use the average of the two parallel sides times the width times the depth.

If you are filling a block wall or a post hole, the volume is still length × width × height (or πr²h for a cylinder), but the shape of the hole is often irregular. In that case, measure the top and bottom dimensions and take an average.

For a concrete calculator that handles multiple shapes, you can use the same formulas manually, or you can use this calculator for each section and sum the results. The key is to keep all measurements in feet before multiplying.

Frequently Asked Questions

How many cubic feet are in a 60-lb bag of concrete?
A 60-lb bag of premix concrete yields about 0.45 cubic feet when mixed with water. That is the standard figure used by most manufacturers, though actual yield can vary slightly with the amount of water added.

How do I convert cubic feet to cubic yards?
Divide the cubic feet by 27. Since 1 yard = 3 feet, 1 cubic yard = 3 × 3 × 3 = 27 cubic feet. For example, 13.33 ft³ ÷ 27 = 0.49 yd³.

What is the formula for concrete in cubic feet?
The formula is Length (ft) × Width (ft) × Depth (ft). If your depth is in inches, divide it by 12 first. For round columns, use π × radius² × height.

How much concrete do I need for a 10×10 slab?
For a 10 ft × 10 ft slab, you need the depth to calculate volume. At 4 inches deep, that is 10 × 10 × (4/12) = 33.33 cubic feet, which is about 1.23 cubic yards.

Should I order extra concrete?
Yes, it is standard practice to add 5–10% to your calculated volume to account for spillage, uneven subgrade, or measurement errors. For a small pour, that might mean one extra bag; for a large slab, it could be a fraction of a yard.

Can I use this calculator for a round column?
No, this calculator is for rectangular slabs and footings. For a round column, use the formula π × (diameter/2)² × height, with all dimensions in feet, to get cubic feet.

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in l | Length | ft | 10 | 0
in w | Width | ft | 4 | 0
in d | Depth | in | 4 | 0
out cf | Concrete volume | cu ft | l * w * (d / 12) | 2
out cy | Concrete volume | cu yd | (l * w * (d / 12)) / 27 | 2
out bags | 60-lb bags (est.) | bags | ceil((l * w * (d / 12)) / 0.45) | 0