Concrete Calculator: Cubic Yards For Slabs And Flatwork

Calculate concrete volume before pricing the pour

Enter the rectangular section’s length and width in feet and depth in inches. The calculator converts depth to feet, computes cubic feet, and divides by 27 to return cubic yards. It also displays 5% and 10% comparison values; those are not universal ordering recommendations.

Use measured dimensions and approved slab details. Thickened edges, footings, ramps, haunches, steps and irregular areas should be calculated as separate shapes and then added. This tool calculates quantity only. For live delivered-cost context, use the concrete price guide.

How to use the calculator

1. Measure the inside dimensions of the forms or the dimensions required by the drawings.
2. Enter length and width in feet and depth in inches.
3. Record the base cubic yards before adding any allowance.
4. Calculate each distinct shape separately; avoid hiding extra depth inside a guessed average.
5. Reconcile the takeoff to thickened sections, blockouts, penetrations and embedded items.
6. Discuss supplier minimums, load rules, batch increments, access, placement rate and returns with the serving plant.
7. Document why the order differs from the measured volume.

Do not reuse a concrete-volume allowance for sheet materials. Wallboard is counted by coverage and layout; use the drywall calculator for that takeoff.

What the calculator includes—and leaves out

NIST defines volume as the measure of three-dimensional space occupied by matter or enclosed by a surface, measured in cubic units. See NIST’s volume reference.

This page applies dimensional volume math only. The calculator does not include a mix design, reinforcement, pump, labor, delivery, short-load, standby, taxes, weather plan, testing or regional ready-mix price. Those belong in the order and cost plan after quantity is established.

The concrete cubic-yard formula

For one rectangular section:

Cubic yards = length in feet × width in feet × (depth in inches ÷ 12) ÷ 27

The first division converts inches to feet. Multiplying the three dimensions in feet gives cubic feet. The second division converts cubic feet to cubic yards because one yard equals three feet and a one-cubic-yard cube is 3 ft × 3 ft × 3 ft, or 27 cubic feet.

For multiple sections:

1. Calculate each rectangular prism separately.
2. Use the correct depth for that section.
3. Add the cubic-yard results.
4. Deduct only true voids that are not filled with concrete.
5. Keep the calculation precision until the final total; round the ordering discussion only after supplier constraints are known.

For nonrectangular geometry, break the work into measurable shapes or use the project designer’s approved quantities. Do not force a complex section into a single slab thickness.

Worked example: 20 by 12 by 4 inches

Use the calculator’s default example: a 20 ft × 12 ft slab at 4 in deep.

1. Convert depth: 4 ÷ 12 = 0.3333 ft.
2. Compute cubic feet: 20 × 12 × 0.3333 = 80 cu ft.
3. Convert to cubic yards: 80 ÷ 27 = 2.963 cu yd.
4. The calculator’s comparison outputs are 3.111 cu yd at 5% and 3.259 cu yd at 10%.

The defensible takeoff baseline is about 2.96 cubic yards before any order adjustment. Do not automatically order either comparison value. First add separate volumes for thickened edges, footings or other shapes, then evaluate measured grade variation, form tolerances, placement plan and supplier rules.

Keep the unrounded calculation in the takeoff so later changes to length, width or depth can be recomputed without compounding rounding.

Ordering allowance: show the reason, not just a percentage

A percentage cannot replace job-specific ordering review. Use this reconciliation:

Quantity	Basis	Keep separate?
Design or measured volume	Geometry and approved depth	Yes
Additional structural shapes	Separate footing, edge, ramp or haunch calculations	Yes
Field/order adjustment	Documented grade, form or placement conditions	Yes
Supplier adjustment	Minimums, increments or service rules confirmed by plant	Yes
Final ordered quantity	Sum accepted by purchaser and supplier	Yes
Delivered/returned quantity	Tickets and field record	Yes

A 5% or 10% display is useful for sensitivity checking: it shows how much a proposed allowance adds. It does not prove that allowance is right. Review the subgrade, forms, dimensions, access and placement sequence, then confirm the order with the ready-mix supplier.

After placement, compare theoretical, ordered, delivered and returned quantities. Update estimating assumptions only when the cause of the difference is known.

Concrete calculator FAQ

What is the concrete calculator formula? Cubic yards = length in feet × width in feet × (depth in inches ÷ 12) ÷ 27.

Why divide by 27? A cubic yard is 3 ft × 3 ft × 3 ft, which equals 27 cubic feet.

Should I automatically add 5% or 10%? No. The calculator shows both as comparisons. Choose any order adjustment from the actual shapes, grade, forms, placement risk and supplier rules, and document the reason.

Does this calculator price concrete? No. It calculates volume only. Obtain a local quote for the mix, delivery and service conditions.

Can I use it for footings? Yes, for a rectangular footing. Calculate each distinct section separately and add the volumes.

What if the slab has thickened edges? Calculate the main slab and the additional thickened volume as separate shapes so the extra depth is visible.

How should I round the result? Keep calculation precision through the takeoff, then discuss the final order with the serving plant because supplier increments and load policies vary.