Drilled piers and caissons, counted honestly, including the overbreak every drilled hole really has. Enter shaft diameter, depth, optional bell and quantity for per-pier and total volumes.
Pier Caisson Calculator
A drilled hole is never the neat cylinder on the drawing, soil ravels and the auger wanders. The waste field here is your overbreak allowance: 10% is a floor, not a ceiling.
Total (with waste/overbreak)
If the holes are already drilled, measure what you actually have, an auger that ‘walked’ one size up raises volume by over 40% (area follows diameter squared). Re-run the numbers with the as-drilled diameter.
A pier caisson calculator treats each pier as a cylinder, π × radius² × depth, plus a frustum if the pier is belled. Six piers at 12 inches diameter and 4 ft deep hold 3.14 ft³ each: 0.70 yd³ in total, 0.77 yd³ with a 10% overbreak allowance, or 35 bags. Enter your shaft and bell above.
How to Calculate Concrete for Drilled Piers
Piers concentrate a building’s loads into holes in the ground, and the concrete estimate lives or dies on two things drawings don’t show: how big the hole actually came out, and whether the bottom is belled. A drilled shaft in soft soil ‘overbreaks’, it caves and ravels wider than the auger, and every extra inch of diameter adds volume with the square of the radius. This pier caisson calculator counts the neat cylinder, the optional bell, and the overbreak allowance that separates estimates from arguments.
The Formula
Bell (ft³, if any) = π × bell height ÷ 3 × (R² + Rr + r²)
Total = (shaft + bell) × quantity × (1 + overbreak %)
R is the bell radius and r the shaft radius in the bell formula. Bells are formed by a belling tool that under-reams the base; the bell height is usually a function of the bell diameter. Straight shafts in stable soil or rock are the ‘0 bell’ case, and their volume is pure cylinder.
Worked Example: Six Deck Piers, 12 in × 4 ft, Straight Shaft
Per pier: π × 0.5² × 4 = 3.14 ft³. Six piers: 18.85 ft³ = 0.70 yd³ exact. With 10% overbreak: 0.77 yd³, or 35 × 80 lb bags for a bag-and-mixer job. This is the classic deck-pier pour, and the classic under-order: people remember ‘a few bags per hole’ from 8 in tubes and meet the 12 in reality (5.2 bags exact per hole) at the store.
Volume per Pier by Diameter (per ft of depth)
| Shaft diameter | ft³ per ft | yd³ per 10 ft | Bags per ft (80 lb) |
|---|---|---|---|
| 8 in | 0.35 | 0.13 | 0.6 |
| 10 in | 0.55 | 0.20 | 0.9 |
| 12 in | 0.79 | 0.29 | 1.3 |
| 16 in | 1.40 | 0.52 | 2.3 |
| 18 in | 1.77 | 0.65 | 2.9 |
| 24 in | 3.14 | 1.16 | 5.2 |
| 36 in | 7.07 | 2.62 | 11.8 |
Read the diameter-squared effect off this table: doubling the diameter quadruples the volume. A hole drilled one size over, 14 in instead of 12 in, quietly adds 36% more concrete. On production pier work, measuring the as-drilled diameter is the whole ballgame.
Overbreak: The Allowance That Is Not Waste
Overbreak is not carelessness, it is geology. Loose sand ravels, soft clay squeezes and bells slump. Ten percent covers clean holes in firm soil; 15–25% is realistic in raveling ground, and drillers who keep a pour log will tell you their local number. Never plan a pier order at exact cylinder volume: a pier that finishes a foot low is a structural defect, not a saving, and the second truck costs more than the allowance ever would.
How to Measure and Calculate: Step by Step
- Confirm shaft diameter, depth and bell from the foundation drawing.
- After drilling, verify diameter and depth in the hole, measure, don’t assume.
- Enter shaft, bell (or 0), quantity and overbreak above.
- Note volume per pier, it is the figure to check against the truck’s meter during the pour.
- Pour each pier continuously and record actual yards per pier; a sudden jump means a cave-in (and a conversation with the engineer).
Worked Example 2: Belled Pier, 18 in Shaft, 36 in Bell, 10 ft Deep
Shaft: π × 0.75² × 10 = 17.67 ft³. Bell at 12 in height: π × 1 ÷ 3 × (1.5² + 1.5×0.75 + 0.75²) = π ÷ 3 × 3.94 = 4.12 ft³. Total per pier ≈ 21.8 ft³ = 0.81 yd³; with 15% overbreak, 0.93 yd³ per pier. The bell added 23% to this pier, bells are bearing area you can see in the volume.
Common Quantities at a Glance: Cheat Sheet
| Job / quantity | Result |
|---|---|
| 12 in pier, 4 ft deep | 3.14 ft³ · 6 bags with allowance |
| Per ft of 12 in shaft | 0.79 ft³ · 1.3 bags |
| Per ft of 24 in shaft | 3.14 ft³ · 5.2 bags |
| 8 piers, 12 in × 5 ft | 1.16 yd³ exact |
| 36 in bell adds (on 18 in shaft) | ≈ 4.1 ft³ per pier |
| Overbreak, raveling soil | Use 15–25%, not 10% |
Practical Tips From the Job
Keep a per-pier pour log: theoretical versus actual tells you about the ground better than any report, and a pier using double its theoretical volume has found a void the engineer needs to know about. Pour piers the day they are drilled where soil or water can degrade the hole, and never leave a belled pier open overnight in soft ground.
Common Mistakes to Avoid
- Ordering exact cylinder volume with no overbreak allowance.
- Assuming the drilled diameter equals the specified diameter.
- Forgetting the bell volume on belled designs.
- Letting water or cave-in debris sit in the hole before the pour.
- Stopping a pier pour mid-shaft and creating a cold joint below grade.
What It Costs and How to Order Smart
Pier concrete is ordinary ready-mix money, the example six-pier job is under a yard, but the delivery pattern is not: a short load for piers, plus a pump or chute work around the site. Bagging 35 bags is genuinely competitive at deck scale; at 24 in shafts and beyond, the per-foot bag counts in the table end the discussion. Compare both with the Concrete Cost Calculator.
Who Uses This Calculator?
Builders laying out foundations take per-pier figures into the field. Contractors reconcile truck meters against the theoretical volume pier by pier. Engineers review pour logs for the overbreak signature of bad ground. Architects rarely count piers, but homeowners building a deck get the first honest answer to ‘how many bags per hole, really?’
Pier Caisson Calculator FAQs
How much concrete is in a 12 inch pier 4 feet deep?
The cylinder is π × 0.5² × 4 = 3.14 ft³, which is 5.2 × 80 lb bags exact, 6 bags per pier with a working allowance. Six such piers total 0.70 yd³ exact, 0.77 yd³ with 10% overbreak. Diameter dominates: a 16 in pier the same depth holds 5.59 ft³, nearly double.
What is overbreak in drilled piers?
Overbreak is the extra volume a real drilled hole takes beyond the theoretical cylinder, because soil ravels, caves or squeezes wider than the auger. Firm ground runs near 10%; loose or wet ground can exceed 25%. It is measured, not guessed, by comparing the truck’s delivered yards against this calculator’s theoretical total, pier by pier.
How does a bell change pier concrete volume?
A bell (an under-reamed, widened base) adds a frustum of concrete: for an 18 in shaft belled to 36 in with a 12 in bell height, about 4.1 ft³, roughly a 23% increase on a 10 ft pier. Enter bell diameter and height in this calculator; leave both at 0 for straight shafts.
How many bags of concrete per pier?
Divide the pier’s cubic feet by 0.60 ft³ per 80 lb bag and round up: a 12 in × 4 ft pier needs 6 bags with allowance; a 12 in × 5 ft needs 7; a 16 in × 4 ft needs 10. The per-foot table on this page lets you scale any diameter and depth before you reach the store.
How deep should deck piers be?
Below the local frost line and onto firm bearing soil, frost depth is set by your climate and code, from under 12 in in warm states to 48 in and more in cold ones. Depth in this calculator is whatever your code and soil require; the volume scales linearly with it, so a frost-driven extra foot on six 12 in piers adds only about 0.17 yd³.
What is the difference between a pier and a caisson?
In US practice the terms overlap: both are drilled, cast-in-place deep foundations. ‘Caisson’ often implies a larger diameter, engineered drilled shaft; ‘pier’ covers everything from a sonotube deck pier to a belled footing. The volume math, cylinder plus optional bell, is identical, which is why one calculator serves both.
Can piers be poured with bagged concrete?
Small straight piers, yes, the deck example is 35 bags for six piers. Large, deep or belled piers belong to ready-mix: the pour must be continuous, placed fast and often tremied below water. Judge by the per-pier bag count and the continuity the design needs, not by habit.
Why did one pier take far more concrete than the others?
A cave-in, a void, an old fill pocket or a buried obstruction that deflected the auger. Log it, tell the engineer, and check the pier’s position and the neighbouring ground. Overbreak concentrated in one pier is information about the site; spread evenly, it is just the soil’s normal allowance.
How accurate is this pier caisson calculator?
The cylinder and frustum geometry is exact; the uncertainty is the hole itself. Measure as-drilled diameters, keep the overbreak allowance honest for your soil, and reconcile against delivered yards pier by pier. On engineered shafts, the geotechnical report’s stated overbreak expectation governs the order.
Related Calculators
Sources used for standards and conversions: National Ready Mixed Concrete Association (NRMCA) guidance on ordering and yield; American Concrete Institute (ACI) practice for mix and placement; Portland Cement Association (PCA) design and control guidance; US manufacturers’ printed yields (80 lb bag = 0.60 ft³). Depths, sizes and spacings set by your local building code govern wherever they differ from the general figures here.
Disclaimer: Results are estimates for planning and ordering. Confirm quantities with your supplier before ordering, and have structural work, footings, walls, columns, beams and anything load-bearing, checked by a licensed engineer.