Post Hole Concrete Calculator

Estimate mixed concrete and premix bags for identical post holes. Subtract the space occupied by each post and account for any concrete below its end.

Dimension unitsChanging units converts the four dimension fields.

Number of identical holesholesRun different-sized holes separately and add their volumes before rounding bags.

Hole cross-section

Hole diameter / square widthinUse the measured finished hole, not the nominal auger size.

Post cross-sectionRectangular and tapered posts are not modeled.

Actual post width / diameterinMeasure your post. The default is 3.5 inches, not a claim that every nominal 4×4 is this size.

Concrete fill depthinOnly the depth filled with concrete. Exclude a gravel bed and any unfilled top section.

Concrete below the post endinPart of the fill depth with no post inside it. Enter 0 when the post reaches the bottom of the concrete.

Extra material allowance%Planning allowance for overdig and losses. Measure the hole and choose an appropriate value.

Bag-yield helperExamples for this specific mix, not all concrete products. The editable yield below is what is used.

Bag yield unitsSwitching this unit converts the yield value.

Mixed concrete yield per bagft³/bagUse the mixed yield printed on the product. Bag weight alone is not enough.

Price per bag (optional)your currencyUses whole bags for the pooled project; excludes labor, equipment and tax.

Calculation results

Estimated material

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How to use this calculator

Choose the approved hole dimensions

This tool estimates material for dimensions you provide. Determine post-hole sizing, frost depth and load requirements separately with the relevant local authority or qualified designer.

Measure the actual post

Select round or square and enter the actual width or diameter. A square post’s diagonal must fit inside a round hole.

Separate concrete from gravel

Enter only the concrete-filled depth. If concrete extends below the post end, enter that depth separately so the post is not subtracted from the bottom layer.

Use the product yield

Choose a listed bag-yield example or enter your product’s mixed yield. The main bag count pools the whole project; the breakdown also shows the larger count if each hole gets separately rounded bags.

Formula and worked example

Round cross-section = π × diameter² ÷ 4
Square cross-section = width²
Concrete per hole = hole area × fill depth − post area × (fill depth − concrete below post)
Total order volume = concrete per hole × number of holes × (1 + allowance ÷ 100)
Bags = round up(total order volume ÷ mixed yield per bag)

For ten round 12-inch holes with 30 inches of concrete fill around 3.5-inch square posts reaching the bottom, each hole holds approximately 1.7508 ft³ of concrete. Ten holes need 17.5082 ft³ before allowance, or 19.2590 ft³ with 10% extra. At 0.60 ft³ per bag, the pooled order is 33 bags. Rounding each hole independently gives 4 bags per hole, or 40 bags; use that only if you cannot share batches between holes.

Example premix yields

Manufacturer-specific approximate yields. Round only after dividing the total project volume by the yield. Check the packaging for the product you buy.
QUIKRETE Concrete Mix 1101	Approximate mixed yield	For 6.0 ft³, before allowance
40 lb bag	0.30 ft³	20 bags
50 lb bag	0.375 ft³	16 bags
60 lb bag	0.45 ft³	14 bags
80 lb bag	0.60 ft³	10 bags

Frequently asked questions

Do you subtract the fence post from the concrete volume?

Yes. The calculator subtracts the actual post cross-section over the part of the concrete depth occupied by the post. It does not subtract the post from a concrete layer below the post end.

How many bags do I need per hole?

Divide that hole’s allowance-adjusted concrete volume by the mixed yield per bag and round up. For several holes, the main result rounds the combined volume only once; separately rounding every hole can buy more bags.

Is concrete fill depth the same as total hole depth?

Not necessarily. A gravel bed or unfilled top portion is not concrete and should be excluded. For example, a 36-inch excavation with 6 inches of gravel and no top gap has 30 inches of concrete fill.

Can I use these defaults to size a footing?

No. The defaults are an arithmetic example, not a structural recommendation. Hole size, embedment, soil, frost exposure, loads and local requirements must be established separately.

Can I use fast-setting concrete or a different bag size?

Yes, if you enter that product’s actual mixed yield in cubic feet or liters. The helper lists QUIKRETE Concrete Mix 1101 yields; it does not prescribe mixing, water addition or installation methods for another product.

Methodology and sources

Uniform straight holes and centered, constant-section posts are assumed. Tapered excavations, irregular overdig, bell footings, reinforcement and an above-grade concrete crown are not modeled. Add measured extra volume separately. This tool does not select a safe footing, approve structural loads or replace local requirements. Follow the product’s mixing and safety instructions.

QUIKRETE Concrete Mix 1101 — technical data sheet — Source for the approximate bag yields in the helper and table.
NIST Handbook 44 — measurement tables — Unit definitions used for inch, foot, yard, centimeter and liter conversions.

Sources checked September 1, 2026. Referenced organizations do not endorse Efficienco. Calculations retain full precision until display; purchase units round up.