| Volume per hole | — |
|---|---|
| Total volume (before waste) | — |
| Total volume (with waste) | — |
| Estimated bags required | — |
| Estimated cost | — |
Typical configurations shown for instant reference. Values adapt to the selected unit system.
| # | Diameter | Depth | Holes | Total volume | With waste | Bag size | Estimated bags |
|---|
Chart displays total volume and estimated bag count for your current inputs.
- Cylinder volume per hole: V = π × (D/2)2 × H
- Imperial inputs: D and H in inches; converted to feet for volume (1 ft = 12 in; 1 ft³ = 28.3168 L)
- Metric inputs: D and H in centimeters; converted to meters for volume
- Total volume: Vtotal = V × holes
- Waste-adjusted volume: Vwaste = Vtotal × (1 + waste%)
- Estimated bags: bags = ceil( Vwaste / yield_per_bag ), where yields are approximate for premix bags
Bag yields vary by brand and mix. Always check the printed yield on the bag for best accuracy.
- Select your unit system.
- Enter the number of post holes, their diameter, and depth.
- Choose a waste allowance to cover spillage, irregular holes, and bulging near the top.
- Select the premix bag size you plan to buy and optionally enter cost per bag.
- Click Calculate to see volume, bags required, and cost.
- Review the chart and example table for context. Export CSV or PDF if needed.
- Round up bag counts in the field to the next full bag and keep a spare for contingencies.
1) Which bag size should I choose?
Pick the size commonly available locally. Larger bags reduce total bags but are heavier to handle. Confirm the printed yield on the packaging.
2) What waste percentage is typical?
Ten percent is a practical default. Increase to 15–20% for rocky soils, irregular auger holes, or significant bell-outs at the top.
3) Do I need gravel at the base?
Many builders add a few inches of compacted gravel for drainage. If you do, reduce the concrete depth accordingly in the calculator.
4) Can I set posts directly in concrete?
Yes, but consider moisture and frost conditions. Some prefer concrete footings with post bases to keep wood above grade.
5) How accurate are the yields?
Yields here are industry-typical estimates for premix bags. Actual yields vary by brand, water content, and compaction. Always verify before purchasing.
6) Should I round up the bag count?
Yes. Bags are sold whole. Rounding up ensures you will not run short and lets you account for unexpected soil collapse or wider holes.
7) Can I switch between imperial and metric?
Absolutely. Toggle the unit system at the top. The example table and bag choices will adjust automatically to match your selection.