← Home & Construction Calculators
Estimate concrete for fence, deck, pergola and sign-post holes. Account for round or square post displacement, repeated holes, waste, bag yield and current supplier prices.
Post-hole formulas
All measurements are converted to feet first. Choose no displacement when concrete is placed before a surface-mounted bracket or when you want a conservative gross-hole estimate.
Worked example
Ten 12-inch-diameter holes, 30 inches deep, containing 4-inch square posts require about 16.86 ft³ net. With 10% allowance, order about 18.54 ft³, or 31 bags yielding 0.6 ft³ each.
Visual guide
Measure the excavated hole after digging. Soil collapse and bell-shaped bottoms can increase volume significantly.
Planning checklist
- Verify property lines, utilities and permit requirements.
- Check frost depth and minimum post embedment.
- Use actual excavated dimensions, not auger labels alone.
- Remove loose soil and standing water as required.
- Brace posts plumb before concrete placement.
- Confirm drainage, slope and top-of-footing finish.
For holes with enlarged bases, calculate the shaft and bell as separate cylinders, then add the volumes.
Frequently asked questions
Should I subtract the post volume?
Yes, when the post is embedded in the concrete. Select its shape and enter the actual cross-section.
How much extra concrete should I order?
Five to fifteen percent is common for clean formed work; irregular soil holes may need more.
Can I use metric dimensions?
Yes. Select centimeters or meters before entering all dimensions.
Does this determine safe footing size?
No. It estimates material only; a qualified designer must determine size and reinforcement.
Common mistakes
- Using radius instead of diameter.
- Ignoring post displacement.
- Using planned rather than excavated dimensions.
- Forgetting bell bottoms and over-excavation.