Post Hole Concrete Calculator

Hole minus post, times holes

The worked example: a 12 in hole is a 1 ft cylinder, so 24 in of depth holds π ⁄ 2 = 1.57 cu ft. A dressed 4×4 (3.5 in per the PS 20 lumber standard) displaces 0.17 cu ft, leaving 1.4 cu ft per hole. Eight holes need 11.21 cu ft; with 10% extra that is 12.33 cu ft — 33 bags of 50 lb mix at the manufacturer's 0.375 cu ft yield, or 21 bags of 80 lb at 0.6 cu ft. Computed at build time by the same engine wired to the form.

The yields that last step divides by — how much placed concrete each of the 40, 50, 60, 80 lb and 25 and 30 kg bags produces, where the figures come from, and why the metric sizes are not scaled from the pound ones — are the subject of the guide Concrete bag yields by size. Read it if a yield is new to you. The rest of this page explains what the post-hole engine does with your dimensions before it reaches the yield, stage by stage, and how to read what comes back.

What the calculator computes, stage by stage

Every estimator on this site follows the five-stage pipeline documented on the methodology page. The post-hole tool runs all five; what makes it different from a slab or footing is the subtraction in the first stage and the fact that the geometry is repeated for a count of identical holes before anything is rounded.

1. Geometry. The hole is a cylinder, π × (diameter ÷ 2)² × depth, with the diameter and depth each divided by 12 so the volume comes out in cubic feet. The post below grade is then subtracted: a square post is a rectangular prism, width × width × depth, and a round post is a second, smaller cylinder. The engine assumes the post is embedded the full depth you entered, so the subtraction uses that same depth. Before it subtracts anything it checks that the post physically fits. A round post must be narrower than the hole; a square post must fit corner to corner, which means its diagonal, width × √2, has to sit inside the hole diameter — 4.95 in for a dressed 4×4, 7.78 in for a dressed 6×6. The net per hole is the hole minus the post, and the net for the job is that figure times the count. From the one unrounded cubic-feet total the engine also reports cubic yards, through the exact 27 cu ft per cubic yard, and cubic meters, through the exact 0.028316846592 m³ per cubic foot.
2. Coverage. The total is divided by a bag yield — the volume of placed concrete one bag produces — once per bag size, so the result shows a count for every size at the same time rather than only the one you selected. The imperial yields are manufacturer figures from the QUIKRETE Concrete Mix (Product No. 1101) product data sheet, "YIELD" section, stated by the manufacturer as approximate: 40 lb → 0.3 cu ft; 50 lb → 0.375 cu ft; 60 lb → 0.45 cu ft; 80 lb → 0.6 cu ft. In metric mode the divisors are the yields Target Products Ltd., the licensed manufacturer of QUIKRETE in western Canada, publishes for its 25 kg (0.43 cu ft) and 30 kg (0.52 cu ft) bags. None of the yields is an editable field, because a typed-over yield would no longer be the manufacturer's.
3. Waste. The engine applies total × (1 + waste ÷ 100) to the cubic-feet total for the whole job, before any rounding. The default of 10% is a trade convention recorded in the site's materials data — the common allowance for spillage and a hole that is not a perfect cylinder — not a manufacturer figure, and the form lets you replace it. The result is the adjusted volume: what the job is expected to consume.
4. Package rounding. For each bag size the engine computes ceil(adjusted ÷ yield). This is the only rounding in the chain and it always rounds up, because a count rounded down leaves the last hole short. It happens once, on the adjusted total, never per hole; the leftover for each size is bags × yield − adjusted, the concrete the packaging obliged you to buy beyond the allowance. When the division lands a hair above a whole number through floating-point noise, the engine treats it as an exact fit rather than adding a phantom bag.
5. User-priced cost. If you type a price per bag, the cost is that price times the rounded count for the one bag size you say you will buy — not for the other sizes, and not for the adjusted volume. The site never supplies or averages a price; leave the field blank and the line does not appear.

Why each input matters

Hole diameter is the input with the most leverage, because the radius is squared. Double the worked example's hole to 24 in while leaving the depth, post, and count alone and the hole becomes 6.28 cu ft — 4 times the 1.57 cu ft of the 12 in hole. It goes in as inches, and it is yours to choose. The manufacturer's 3×-post-width guidance shown under the form works out to 10.5 in for a dressed 4×4 and 16.5 in for a dressed 6×6; the worked example's 12 in hole is a user's choice, not something the tool applied.

Concrete depth moves the volume in direct proportion — twice the depth, twice the concrete — and it is the depth the concrete will fill, not the depth of the hole you dug. If you place gravel in the bottom of the hole, as the guidance under the form describes, the concrete starts on top of it, so subtract the gravel before you type the depth. The gravel-base pair in the examples below shows what the difference costs in bags. Depth goes in as inches, which is where the unit slip described further down comes from.

Post shape and size set the subtraction. Enter the post's actual size: a nominal 4×4 is 3.5 in square and a nominal 6×6 is 5.5 in, the dry dressed sizes in Table 3 of PS 20-20, the American Softwood Lumber Standard, and the form defaults to the 4×4 figure with the nominal size labeled. Rough-sawn or non-standard stock is typed in as measured. A round post wants its diameter. Because the post is a fraction of the hole, a small error in the post size moves the result only a little; the engine's refusal of a post that cannot fit matters more, and is explained under the misreadings.

Number of holes must be a whole number of at least one. The engine multiplies the per-hole net by it and then runs waste and rounding on the total, which is why one run for the whole job buys fewer bags than one run per hole added together. All the holes in a run are identical; holes of two sizes are two runs, and the cleanest way to combine them is to add the adjusted cubic-feet figures and take that volume to the concrete bag calculator so the round-up still happens once.

Waste multiplies the total, so it moves the adjusted figure proportionally and the bag counts in steps. On a small job a change of a few percent often changes nothing because the round-up absorbs it; then a further change tips one size to the next bag. The percentage is a convention to start from, and zero is accepted when your depth already carries a margin.

Bag size and price touch only the last line. Every size is counted whichever one you select; the selection decides which count the price multiplies. Changing either after the fact never disturbs the volumes above.

The units toggle changes what the fields expect and how the result is presented, not what is computed. In metric mode the diameter, depth, and post size are centimeters, each divided by the exact 2.54 cm per inch before the engine sees them, the headline line leads with cubic meters, and the bag rows are the kilogram sizes with their own published yields.

The figures this page relies on

• Bag yields, imperial — manufacturer figures from the QUIKRETE Concrete Mix (Product No. 1101) product data sheet, "YIELD" section, stated as approximate: 40 lb, 0.3 cu ft; 50 lb, 0.375 cu ft; 60 lb, 0.45 cu ft; 80 lb, 0.6 cu ft. Held in the site's materials data and pinned by tests; not editable in the form.
• Bag yields, metric — manufacturer figures from Target Products Ltd., the licensed manufacturer of QUIKRETE in western Canada: 25 kg, 0.43 cu ft; 30 kg, 0.52 cu ft. Kept as a separate table, never derived from the pound figures.
• Dressed post sizes, 3.5 in for a 4×4 and 5.5 in for a 6×6 — the minimum dry dressed sizes in Table 3 of PS 20-20, the American Softwood Lumber Standard published by NIST. The 4×4 figure is the form's default and is adjustable.
• Hole 3× the post width — the manufacturer's installation guidance from QUIKRETE's "Setting Posts in Concrete" project guide, together with its depth and gravel suggestions quoted under the form. Shown as a labeled suggestion only; never applied to your entries, and not a building code.
• Default waste, 10% — a common trade convention recorded in the site's materials data, where it is labeled a convention because no primary source publishes a universal figure. Adjustable in the form.
• Inches per foot, 12; cubic feet per cubic yard, 27; centimeters per inch, 2.54; cubic meters per cubic foot, 0.028316846592 — definitions, exact.
• The post diagonal, width × √2 — arithmetic, used only for the fit check.

Worked examples

Every figure in this table was produced at build time by the same engine that runs the form above, with the waste field at its 10% default. The inputs are chosen to show different behaviours of the pipeline; the metric run follows in prose because it counts against the kilogram bags.

Four 6×6 posts. An 18 in hole 36 in deep holds 5.3 cu ft, and a dressed 6×6 at 5.5 in displaces 0.63 cu ft of it. The net per hole is 4.67 cu ft, 18.68 cu ft for the four, 20.55 cu ft adjusted, and 35 bags of 80 lb mix or 55 of 50 lb. The 18 in hole clears the 6×6's 7.78 in diagonal with room to spare; the engine would refuse one that did not.

Twelve round posts. With the shape set to Round, a 4 in post in a 10 in × 30 in hole is a cylinder inside a cylinder. The hole holds 1.36 cu ft, the post removes 0.22 cu ft, and the net per hole is 1.15 cu ft; twelve holes need 13.74 cu ft net and 15.12 cu ft with the allowance, which rounds to 26 bags of 80 lb. The only check on a round post is that its diameter is smaller than the hole's.

The gravel-base pair. Fourteen 10 in holes are dug 30 in deep, and 6 in of gravel goes in first, so the concrete fills 24 in. Entered correctly, each hole takes 0.92 cu ft around a 4×4, the job 12.89 cu ft net, and the count is 24 bags of 80 lb. Type the dug depth instead and the engine, computing exactly what it is given, reports 1.15 cu ft per hole and 30 bags — 6 bags of concrete for space the gravel already fills. The tool has no gravel field because gravel is not concrete; the depth you enter is the whole of what it knows.

A single hole. One 8 in × 20 in hole around a 4×4 is 0.58 cu ft less 0.14 cu ft, or 0.44 cu ft net and 0.48 cu ft adjusted. The count is a bag or two whichever size you pick: 1 bag of 80 lb holds 0.6 cu ft and 2 bags of 40 lb hold 0.6 cu ft, leaving 0.12 and 0.12 cu ft spare against the 0.48 cu ft adjusted need. On a job this small the rounding, not the allowance, governs the purchase, and the bag rows are worth reading side by side.

The priced run. Twelve 12 in × 30 in holes around 4×4 posts need 21.01 cu ft net and 23.11 cu ft adjusted, which is 39 bags of 80 lb mix with 0.29 cu ft spare. With $8.00 per bag typed into the price field and the 80 lb size selected, the cost line reads $312.00 — the rounded count times the price, nothing else. That price is a placeholder chosen only to show the arithmetic; the site holds no concrete prices, and the cost is computed on the rounded count because the rounded count is what you buy.

The metric run. In metric mode, ten 30 cm × 60 cm holes around an 8.9 cm post reach the engine as 11.81 in × 23.62 in holes around a 3.5 in post, each conversion exact. The net per hole is 1.33 cu ft and the job 0.38 m³ (13.3 cu ft); the allowance takes it to 0.41 m³, and against the kilogram table that is 29 bags of 30 kg at 0.52 cu ft each, or 35 bags of 25 kg at 0.43 cu ft. The result panel leads in cubic meters, but the path through the engine is the one the imperial examples took.

How to read the result

The headline line is the volume to mix: Mix N cu ft of concrete, or cubic meters in metric mode. Beneath it, Per hole shows the three geometry figures — hole, post, and the difference — so a typo in a dimension is visible before it is multiplied; the next line names the count and the percentage and gives the adjusted total, what the job is expected to consume; and then one row per bag size gives the whole bags that size needs. Read the per-hole line to check your geometry, the adjusted line to check your allowance, and the bag row for the size you will carry to place the order. A cost line appears only when a price is entered, and it multiplies the selected size's count. The copy button places the net volume and every bag count on the clipboard as plain text.

Common misreadings

Depth in feet. Depth goes in as inches. Type the worked example's 24 in as 2 — feet — and the engine sees a 2 in ring of concrete around each post: 0.93 cu ft for the eight holes instead of 11.21, exactly 12 times too little, and 3 bags of 50 lb mix instead of 33. On the slab calculator the same slip runs the other way and inflates the order; here it starves it, because the field that takes inches is the one you are likely to think of in feet. A bag count that looks far too small for the number of holes usually means the depth field.

Diameter in feet. Type the 12 in hole as 1 and the engine sees a 1 in hole, which a 3.5 in post cannot fit; rather than subtract a post that could not be there, it refuses with the message "The post is too wide for this hole — its corner-to-corner diagonal must fit inside the hole diameter." If you see that message with a sensible post, check the diameter's unit first.

Dug depth instead of concrete depth. The gravel-base pair above is the whole story: the engine fills whatever depth you give it, and gravel is not concrete. Enter the depth from the top of the gravel to the top of the pour.

Nominal post size. Type the 4×4 as 4 in instead of its dressed 3.5 in and the subtraction grows: the net per hole falls to 1.35 cu ft from 1.4, and the eight-hole job counts 32 bags of 50 lb instead of 33. The error is small and it under-buys, which is the wrong direction; the default is the dressed figure for that reason.

Skipping the subtraction. Counting the eight gross cylinders with no post removed puts 12.57 cu ft through the bag engine — 13.82 cu ft adjusted and 37 bags of 50 lb against the 33 the tool reports. The over-buy is the post volume times the count, padded by the allowance and rounded, and it grows with the post size.

Rounding per hole. Running one hole and multiplying the bags by the count rounds eight times instead of once. The worked example's single hole needs 5 bags of 50 lb after rounding; eight of those is 40, where the whole job rounds to 33. Enter the count and let the engine round the total.

Where the model stops

The engine models a true cylinder with the post embedded its full depth and assumes that shape is what gets dug. It does not model a hole that is wider at the top than the bottom, an irregular side, a post set shallower than the concrete, or concrete that stays in the mixing tub — those are among the reasons real consumption differs from the geometry, and the waste allowance is the single flat percentage you are given to fold them into. It has no gravel field, because the depth you enter already describes what the concrete fills. It estimates concrete quantity and nothing else: the gravel itself, bracing, mix and curing, and delivery minimums are outside the model, as they are for every estimator on the site. And, as the note under the form says, it decides nothing structural — hole diameter, embedment depth, and frost depth for any post that carries a load, and anything a permit depends on, are questions for your building department or an engineer, and the dimensions you enter should already carry that review. This page estimates material quantities only. The methodology page lists the simplifications every estimator shares.

Fences, footings, and the rest

Setting a whole fence line? The fence calculator counts posts, rails, and pickets and reuses this exact hole math for the setting concrete. Strip footings are the concrete footing calculator's job, formed piers the concrete column calculator's, and the concrete bag calculator compares every bag size for any volume. For the yields behind the bag rows, see concrete bag yields by size; for the volume units, cubic feet, cubic yards, and ordering concrete; for the allowance, waste factors, explained; or browse all guides.

Frequently asked questions

Dimensions never leave your browser. Sources and the estimating pipeline are documented on the methodology page.