Direct concrete answer
How much concrete for a 20 foot footing?
A 20 ft continuous footing that is 24 inches wide and 12 inches deep requires 1.49 cubic yards of concrete, or 67 bags of 80-lb premix before field waste. With 10% overage, order about 1.7 cubic yards.
Quick answer
| Project dimensions | 20 ft long x 24 in wide x 12 in deep |
|---|---|
| Area or count | 20 linear ft |
| Volume | 40.00 cubic feet |
| Volume | 1.49 cubic yards |
| Metric volume | 1.13 cubic meters |
| 80-lb bags needed | 67 bags |
| 60-lb bags needed | 89 bags |
| 50-lb bags needed | 107 bags |
| Ready-mix order with 10% overage | 1.7 cubic yards |
| Ready-mix planning cost at $150/yd3 | $244 |
| Bagged planning cost at $5/80-lb bag | $335 |
Calculation steps
- Convert depth to feet: 12 inches / 12 = 1.000 ft.
- Calculate cubic feet: 20 ft x 2 ft x 1.000 ft = 40.00 ft3.
- Convert to cubic yards: 40.00 ft3 / 27 = 1.49 yd3.
- Add waste for ordering: 1.49 yd3 x 1.10 = 1.7 yd3. Round according to the supplier's ordering increment.
This is the exact math behind the answer, but the field order should reflect the actual formed dimensions, the base condition, and the supplier's minimum order policy. Footing trenches are rarely perfect. Measure actual excavated width and depth after digging, especially where sides crumble or the trench steps down.
Number of bags by size
Bag counts use common premix yields and round up to whole bags. Always check the yield printed on the exact product before buying because specialty mixes can differ.
| Bag size | Common yield | Calculation | Bags to buy |
|---|---|---|---|
| 80-lb bag | 0.6 ft3 | 40.00 / 0.6 | 67 |
| 60-lb bag | 0.45 ft3 | 40.00 / 0.45 | 89 |
| 50-lb bag | 0.375 ft3 | 40.00 / 0.375 | 107 |
Cost snapshot
At a planning price of $150 per cubic yard, the waste-adjusted ready-mix concrete is about $244 before short-load, delivery, pump, labor, forms, base stone, reinforcement, tax, or finish upgrades. The 80-lb bag material estimate is about $335 before mixer rental or labor.
Cost is where small concrete jobs surprise people. A short-load fee can make the delivered price look high, while bagged concrete can look cheap until the crew has to mix 67 heavy bags fast enough to place and finish one continuous surface. Use the material number as a quote starting point, not as the final installed price.
Ready-mix vs bagged concrete
Footings are commonly ready-mix jobs when the trench must be filled continuously around reinforcement.
As a rule of thumb, ready-mix becomes easier once a project is near or above one cubic yard. This page's base volume is 1.49 cubic yards, so the practical choice depends on access, crew size, weather, finish timing, and whether the supplier charges a short-load fee. If the pour must be continuous, the truck often reduces risk even when the invoice includes a delivery charge.
Recommended PSI and reinforcement
| Use | Typical planning PSI | Notes |
|---|---|---|
| Patio or walkway | 3,000 PSI | Light foot traffic on compacted base. |
| Garage floor | 3,500 PSI | Verify vapor barrier, joints, and reinforcement. |
| Driveway | 3,500 to 4,000 PSI | Use higher strength for trucks, RVs, or weak soil. |
| Footings | 3,000 to 4,000 PSI | Plan and local code control final mix. |
Many residential footings are specified around 3,000 to 4,000 PSI, but the plan and local code control the final mix.
Footing reinforcement, cover, and lap lengths are design items. Estimate the concrete quantity here, then verify the structural detail locally.
Project-specific notes
This answer is for a 20 ft continuous footing that is 24 inches wide and 12 inches deep. The geometry is straightforward, but the site details decide whether the estimate is clean in the field. Before ordering, compare the calculated dimensions against the actual form layout, the base depth, and the planned finish elevation.
- A short footing can look simple, but over-excavated corners and uneven trench bottoms quickly change volume.
- Use actual trench measurements after excavation, not only the drawing width.
- If the project includes thickened edges, a landing, a flare, a step, or a separate footing, calculate that concrete separately and add it to the base result.
Common mistakes
- Measuring the footing from the drawing after the trench has been over-dug.
- Ignoring steps, widened pads, or thickened areas at load points.
- Treating volume estimating as footing design. Soil bearing, frost depth, and reinforcement still need review.
The expensive mistake is running short during placement. The second most expensive mistake is ordering more concrete than the site can place before it starts setting. The best order is not the smallest number; it is the number that fits the measured work, the crew, the truck access, and the supplier's rounding policy.
What to say when ordering
I am estimating a 20 ft continuous footing that is 24 inches wide and 12 inches deep. My calculated volume is 1.49 cubic yards, and I want to plan around 1.7 cubic yards with 10% overage. Can you confirm the order size, mix strength, short-load fee, delivery charge, chute reach, and whether this job needs a pump or different placement method?
Related calculations
Sources and methodology
BuilderCalc uses standard geometric volume formulas, the 27 cubic feet per cubic yard conversion, common premix bag yields, and planning cost ranges that should be verified with local suppliers.