Estimate concrete for a uniform rectangular ring footing using its centerline length and width. The example 40 × 30-foot centerline rectangle with a 16-inch width and 8-inch concrete thickness needs 5.07 cubic yards with a selected 10% allowance, or 229 bags at 0.6 cubic feet per bag. This estimates volume, not footing size, burial depth or structural suitability.
Compare the footing quote on its own scope
Use our concrete cost guide to separate ready-mix material rates from installed slab prices and bagged-mix examples. An installed slab rate per square foot is not automatically a footing quote. Price the specified mix and billable order quantity, then identify delivery minimums, access, pumping and placement costs that are not already included.
Keep excavation, forms, reinforcement, inspections and any separate walls or pads visible in the quote. This calculator only estimates the selected rectangular concrete ring. It does not price the complete foundation or approve its design. Do not add bag costs or another allowance on top of a contractor quote that already includes those items.
Use centerline dimensions, not an arbitrary building outline
The length and width define the rectangle through the middle of the concrete strip. For a uniform-width rectangular ring, centerline perimeter × strip width × concrete thickness gives the ring volume. Outside dimensions, inside dimensions and wall dimensions are different measurement bases. This calculator cannot infer which one your plan uses.
If you know the actual outer and inner concrete rectangles, a separate check is (outer area − inner area) × thickness. For a valid uniform ring, centerline dimensions are the averages of corresponding outer and inner lengths. Do not count overlapping corner blocks twice or apply an outer perimeter as if it were the centerline.
Concrete quantity formulas
Centerline perimeter = 2 × (length + width).
Net cubic feet = perimeter × footing width in inches / 12 × concrete thickness in inches / 12.
Adjusted cubic yards = net cubic feet × (1 + allowance / 100) / 27.
Bags = adjusted cubic feet / actual bag yield, rounded up. These are our geometry formulas. Use full precision until the final whole-bag count.
Example bag yields
QUIKRETE Concrete Mix No. 1101 lists approximate yields of 0.60 cu ft for 80 lb, 0.45 cu ft for 60 lb and 0.375 cu ft for 50 lb bags.1 Those support the dropdown examples, not every concrete mix with the same bag weight. Verify the exact mix yield and its suitability for the specified work.
Quantity chart: selected 16-inch width and 8-inch thickness
Our calculated scenarios use a 10% allowance and 0.6-cu-ft bag yield. These dimensions are math inputs, not recommended footing sizes.
| Centerline (ft) | Run (ft) | Net yd³ | With 10% (yd³) | Bags |
|---|---|---|---|---|
| 20 × 20 | 80 | 2.634 | 2.897 | 131 |
| 30 × 20 | 100 | 3.292 | 3.621 | 163 |
| 40 × 30 | 140 | 4.609 | 5.070 | 229 |
| 50 × 40 | 180 | 5.926 | 6.519 | 294 |
Three checked examples
1. Default centerline rectangle
40 × 30 ft gives 2 × (40 + 30) = 140 ft of centerline. At 16 inches wide and 8 inches thick: 140 × 16 / 12 × 8 / 12 = 124.444444 cu ft. With 10%: 136.888889 cu ft = 5.069959 yd³. Divide by 0.6: 228.148148, rounded up to 229 bags.
2. Check outer minus inner area
An outer concrete rectangle of 21 × 16 ft and an inner rectangle of 19 × 14 ft gives 336 − 266 = 70 sq ft of concrete plan area. At 0.5 ft thick, volume is 35 cu ft. Its 20 × 15-ft centerline rectangle has a 70-ft perimeter; 70 × 1-ft strip width × 0.5-ft thickness gives the same 35 cu ft. With zero allowance and 0.6-cu-ft yield, 35 / 0.6 rounds up to 59 bags.
3. Independent metric inputs
A 10 × 8-m centerline rectangle has a 36-m perimeter. At 40 cm wide and 20 cm thick: 36 × 0.40 × 0.20 = 2.88 m³ net. A selected 10% gives 3.168 m³. Using 0.6 cu ft per bag, equal to 0.016990108 m³, the count rounds up to 187 bags. Metric display may convert rounded US figures; keep the unrounded quantity for the bag calculation.
Is concrete thickness the same as burial depth?
No. Thickness is the vertical concrete cross-section. Depth below grade, frost protection, soil bearing, reinforcement and loads belong to the approved design, not this volume calculation.
Can I include interior footings?
Not as part of this one rectangular perimeter. Take off the actual non-overlapping sections separately and avoid double-counting intersections.
Does a bag count approve the concrete mix?
No. Yield only converts volume to package count. Confirm the specified material and placement requirements separately.
Sources
- QUIKRETE: Concrete Mix No. 1101 data sheet (manufacturer approximate bag yields)
What you’ll need
A plain checklist. Needs vary by project, so follow the instructions on your materials.
- Forms and stakes – hold the concrete.
- Rebar and ties, if your plan calls for it – structural footings follow local code.
- Level and transit – footings need to be level.
- Concrete – see the concrete calculator.
- Local permit – most foundation work needs one. Check with your building office.
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Estimates only. Confirm quantities and product details with your supplier or installer before you order. This is not professional advice.