Calculation method
Compute each cylindrical hole volume, subtract the volume occupied by the rectangular post over its embedded length, multiply by the number of holes, and add the chosen allowance. Divide by the published approximate yield for the selected QUIKRETE Concrete Mix No. 1101 bag and round up.
Worked example: input to purchase quantity
Four 12 in diameter, 24 in deep holes with actual 3.5 × 3.5 in posts embedded 24 in require about 5.603 ft³ net. A 10% allowance gives about 6.163 ft³; at 0.60 ft³ per 80 lb bag, buy 11 bags.
This calculator supports post-hole concrete bag purchasing. Enter round hole diameter and filled depth, actual rectangular post width and depth, embedded length, number of identical holes, bag weight, and a waste allowance. The arithmetic uses the values you provide and displays the purchase result alongside the intermediate amount. The output is intended for a measured planning worksheet; it does not replace a project drawing or product specification.
Measure and choose the correct input
Measure the inside diameter and filled depth of each hole, plus the actual size and embedded length of the post. The post embedment cannot exceed the concrete-filled depth and its rectangular diagonal must fit inside the round hole. Non-round holes, tapered excavation, bell footings, gravel bases, and above-grade forms need separate volume treatment.
How the estimate is calculated
Net cubic feet per hole = [π × (diameter ÷ 2)² × depth − post width × post depth × embedded length] ÷ 1728; apply hole count and allowance, then divide by bag yield and round up. A 12 in by 24 in cylinder is π × 6² × 24 = about 2,714 cubic inches. A 3.5 × 3.5 in post embedded 24 in occupies 294 cubic inches. Net per hole is about 1.401 ft³, four holes about 5.603 ft³, and 10% extra about 6.163 ft³. Dividing by 0.60 ft³ per 80 lb bag and rounding upward gives 11 bags.
Verify the order
Check hole depth after any gravel base and verify which portion is actually filled with concrete. Measure the real post size; a nominal 4×4 is often smaller than four inches. Confirm that the store stocks the selected weight of QUIKRETE 1101 and compare its current yield with the data sheet. Keep a little practical handling margin but do not substitute a slab-volume estimate for the hole geometry.
Where the model ends
The yield table belongs specifically to QUIKRETE Concrete Mix No. 1101: 40 lb 0.30 ft³, 50 lb 0.375 ft³, 60 lb 0.45 ft³, and 80 lb 0.60 ft³, approximately. Other mixes or labels require their own yield. Keep that limitation visible when using the quantity for a real purchase.
Keep a traceable worksheet
A useful order begins with a dated sketch, a product name, and dimensions recorded in their original units. Mark each repeated element so that its count can be checked against the site. Record where each figure came from: a tape measure, a plan, a supplier listing, or a design detail. This makes later changes auditable. If two sections differ, estimate them separately before adding the purchase quantities. A single average may hide a small but important difference at a boundary.
Read the result in stages
The calculator shows intermediate quantities so the final purchase count can be checked. Work through the displayed values in order instead of treating the last number as a guarantee. Keep full precision during the calculation and round only where the model says a purchasable whole item is required. A value just over a whole item still requires another item. If the answer changes after switching units, check whether converted entries were rounded in the form or whether a product-specific unit was assumed.
Separate quantity from design
This worksheet estimates material amounts from chosen project inputs. It cannot inspect the site or decide what assembly is safe and suitable. Structural requirements, local rules, product instructions, drainage, loads, and installation methods belong in the project design. Resolve those matters before treating a material estimate as a purchase list. If the design changes, update the dimensions and parameters and calculate again. Do not use an arbitrary extra percentage to repair an incorrect geometry or unsuitable specification.
Check the seller before paying
Compare the measured result with the unit in which the seller actually quotes and sells material. Check available stock sizes, package quantities, delivery constraints, return terms, and whether a listed size is nominal or actual. A quantity that is mathematically correct can still produce a wrong order when a product is sold by a different unit. Save the product page or label used for the estimate. If the listed yield, dimension, or unit price changes, revise the worksheet rather than reusing an old purchase count.
Product and method reference
Reference limits and default yields are identified separately from user-entered values; verify the adopted local code and current product label. QUIKRETE Concrete Mix No. 1101 data sheet.
Calculation and editorial review: November 3, 2026. Arithmetic runs in your browser; project inputs are not submitted to TwoCut.