Estimating strip concrete volume without double-counting corners

By Kyle Bell · Reviewed November 9, 2026

A long rectangular strip has the same volume formula as a slab: length multiplied by width multiplied by depth. The extra challenge for a continuous perimeter is avoiding overlapping corners. This worksheet estimates material for dimensions supplied by a plan or qualified professional; it does not choose a footing size or establish structural adequacy.

Calculate a simple straight strip

For a total non-overlapping run of 80 feet, a width of 16 inches, and a depth of 8 inches, convert the cross-section to feet. Volume is 80 × (16 ÷ 12) × (8 ÷ 12) = approximately 71.111 cubic feet. That is approximately 2.634 cubic yards before an allowance.

With an example 10% allowance, the requirement becomes approximately 78.222 cubic feet. Using a selected yield of 0.60 cubic foot per bag gives approximately 130.37 bags, rounded up to 131. The QUIKRETE No.1101 sheet supplies that approximate yield for its 80-pound package; other products need their own stated values.

Watch how a perimeter length was measured

A rectangular concrete ring with uniform width can be calculated as outer rectangle area minus inner rectangle area, multiplied by depth. Suppose its outside dimensions are 20 by 16 feet, its uniform strip width is 1 foot, and its supplied depth is 8 inches. The inside dimensions are 18 by 14 feet.

Outer area is 320 square feet and inner area is 252, leaving 68 square feet of concrete footprint. Volume is 68 × 8 ÷ 12 = approximately 45.333 cubic feet. This subtraction accounts for each corner once without requiring a special correction after the fact.

Why the outside perimeter can overcount

The outside perimeter in that example is 72 feet. Multiplying 72 × 1 × 8 ÷ 12 gives 48 cubic feet, which is approximately 2.667 cubic feet too much for the stated ring. The outside-length method effectively includes overlapping corner pieces.

For this uniform rectangular ring, the centreline dimensions are 19 by 15 feet. The centreline perimeter is 68 feet, and 68 × 1 × 8 ÷ 12 produces the correct volume. Do not mix an outside measurement for one side with centreline measurements for the others. If the shape or cross-section changes, break it into clearly defined non-overlapping components instead.

Keep different depths separate

A thickened section, intersecting strip, or stepped detail may require its own component volume. Identify what the drawing includes and subtract shared space only once. Never assume the geometric simplification is a substitute for understanding the approved design detail.

NIST provides the unit conversion reference for switching between cubic feet, yards, and metres. After conversion, apply your selected allowance and the actual supplier's order increment separately.

Record whether each length is outside, inside, centreline, or an isolated segment. Keep the dimensioned sketch with the bag or ready-mix result. That small notation makes the estimate far easier to check than a single “total linear feet” value with no measurement convention attached.

Sources & further reading

• QUIKRETE Concrete Mix No.1101 — Technical data sheet
• NIST Guide to SI, Appendix B.8 — Conversion factors

Source material checked November 9, 2026. This guide is an original explanation; source publishers do not endorse this website.