Fence post concrete calculator
Fence length, spacing, hole and post size: you get the number of holes, concrete per hole and bags of dry mix to buy.
Purchase quantity
0.706 yd³
Net measurement: 0.641 yd³ · Waste 10 %
- Bags of dry concrete mix45 bag
- Holes (one per post)13 unit
- Concrete per hole0.052 yd³
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How it's calculated (visible formula)
hole = π/4 × D² × depth − post² × depth; bags = volume × (1 + waste) ÷ bag yield
- Net geometric measurement, no waste.
- Adjustable waste % for cuts and breakage.
- Purchase rounded up to sellable units (boxes, bags, buckets, boards).
How to calculate it, step by step
- Work out posts from fence length and spacing.
- Set hole diameter and depth.
- Concrete per hole = hole volume minus post volume.
- Multiply by holes and add 10% waste.
- Divide by bag yield for the bag count.
Worked example: 100 ft, 4×4 posts every 8 ft, 12 × 24 in holes
- 13 posts
- ≈ 0.0376 m³ (1.3 cu ft) per hole
- ≈ 0.54 m³ with 10% waste
→ About 45 bags of 55 lb (or 30 of 80 lb).
Common mistakes
- Not subtracting the post volume.
- Digging holes too narrow (less than 3× the post width).
- Filling flush instead of crowning the top to shed water.
Pro tips
- Dampen very dry holes before pouring.
- Plumb posts on two faces and brace while curing.
- Wait 24–48 h before hanging rails or fabric.
- Use fast-setting mix to keep moving the same day.
From calculation to quote
Add labour, overhead and your margin, and generate the professional PDF.
FAQs
- How many bags per fence post?
- For a 12 × 24 in hole, about 2 bags of 80 lb.
- How wide should the hole be?
- About three times the post width.
- How much does an 80 lb bag make?
- About 0.6 cu ft (0.017 m³).
- Can I pour dry concrete into the hole?
- Fast-setting mix works for light posts; otherwise mix it.
- How long does post concrete take to set?
- Fast-setting 20–40 minutes; regular mix 24–48 h before loading.
- Do gate posts need bigger footings?
- Yes — deeper and wider to carry the gate's weight and swing.
Sources and standards
Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.