Concrete Calculator

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How to Use the Concrete Calculator

1. 1

   Pick the project type

   Choose the Slab/Patio, Footing, Column or Stairs tab at the top of the calculator.

2. 2

   Enter the dimensions

   Type length and width in feet. Slab thickness is picked from a list (3, 4, 5, 6 or 8 inches); footing depth and column diameter are entered in inches, column height in feet.

3. 3

   Add counts for columns or stairs

   For columns, set how many identical columns you are pouring. For stairs, enter the width in feet, the rise and run per step in inches, the number of steps and an optional top landing depth in feet.

4. 4

   Read the results

   The results update as you type: cubic yards and cubic feet, the volume with 10% waste added, bag counts for 40, 60 and 80 lb bags (before waste), and the volume in cubic meters.

How the concrete calculator works

Every shape is converted to cubic feet (inches divided by 12), then divided by 27 for cubic yards. Cubic meters are cubic feet times 0.0283168.

Slabs and footings are boxes: length times width times depth. Columns use pi times the radius squared times the height, times the number of columns. Stairs are treated as solid steps on grade: step 1 is one rise tall, step 2 is two rises and so on, each one run deep, with an optional landing at the height of the top step.

For bags, divide cubic feet by the bag yield. Premixed concrete yields about 0.30 cu ft per 40 lb bag, 0.45 cu ft per 60 lb bag and 0.60 cu ft per 80 lb bag. Always round up. The bag counts the tool shows are for the exact volume; the 10% waste is applied only to the cubic yards figure.

Cubic yards = (length ft × width ft × depth in ÷ 12) ÷ 27
Column cubic feet = 3.1416 × (diameter in ÷ 24)² × height ft × number of columns
Bags = cubic feet ÷ bag yield (0.30, 0.45 or 0.60 cu ft), rounded up

Worked example: a 10 x 10 ft patio, 4 inches thick

At that size, ready-mix is usually easier. Mixing more than 50 heavy bags by hand is a long day, and early batches can start to set before the last ones are placed.

• Convert the thickness to feet: 4 ÷ 12 = 0.333 ft.
• Volume in cubic feet: 10 × 10 × 0.333 = 33.33 cu ft.
• Convert to cubic yards: 33.33 ÷ 27 = 1.23 cubic yards.
• Add 10% for waste: 1.23 × 1.1 = 1.36 cubic yards to order.
• 80 lb bags: 33.33 ÷ 0.6 = 55.6, so 56 bags. With 10% waste added to the volume first (33.33 × 1.1 = 36.67 cu ft): 36.67 ÷ 0.6 = 61.1, so 62 bags.
• 60 lb bags: 33.33 ÷ 0.45 = 74.1, so 75 bags. With waste: 36.67 ÷ 0.45 = 81.5, so 82 bags. (Adding 10% to the rounded 75 bags instead gives 83; either is fine, but add waste to the volume before rounding for the tighter number.)

Bags of concrete for common slab sizes

Square slabs at 4 in (patios, walks, shed pads) and 6 in (driveways, heavier pads). Bag counts are rounded up; the "+10%" column adds waste to the volume before rounding.

Bags per cubic yard

Yields vary a little by brand and mix, so if your bag label gives a different yield, divide your cubic feet by that figure.

Concrete for post holes and Sonotubes

Use the Column tab for round holes and form tubes. Figures below are per hole, treating the hole as an empty cylinder, rounded up to whole bags.

• When you enter several columns, the calculator rounds the total, not each hole. Four 12 in × 4 ft tubes total 12.57 cu ft, which it shows as 21 bags of 80 lb; if you mix each hole separately, buy 6 per hole (24 bags).
• A 4x4 post displaces about 0.085 cu ft per foot of depth, so a 3 ft deep 10 in hole needs about 1.38 cu ft rather than 1.64. It rarely saves a bag per hole, but adds up on long fence runs.
• Hand-dug holes are rarely perfect cylinders and usually take more than the math says, so keep a spare bag on site.

Footings, steps and other projects

Figures below are the raw volume before waste. Bag counts use 0.6 cu ft per 80 lb bag and 0.45 cu ft per 60 lb bag, rounded up.

Edge cases the simple formula misses

• Uneven or sloped ground: measure the excavation depth at several points and use the average. A slab planned at 4 in that averages 4.5 in needs 12.5% more concrete, which wipes out a 10% waste allowance.
• Thickened edges: slabs often have a deeper perimeter (for example 8 to 12 in wide and twice the slab depth). Calculate the edge as a separate footing and add it.
• Base under the slab: most slabs sit on 4 in or more of compacted gravel. Work that out with the gravel calculator; it does not reduce the concrete volume, but it does lift your form height.
• Stairs: the calculator assumes solid steps sitting on the ground. Steps poured against an existing slope or over fill take less concrete, so measure what you actually have to fill.

Ordering tips and common mistakes

• Order 5 to 10% extra. Form bulges and spillage eat more concrete than people expect, and running short mid-pour leaves a weak cold joint.
• Round bag counts up after adding waste. The calculator adds 10% to the cubic yards figure only, so multiply the bag count by 1.1 yourself, or use the "+10%" column above.
• Ready-mix is sold by the cubic yard, often in quarter or half yard steps, and many suppliers charge a short-load fee on small orders. As a rule of thumb, bags make sense for jobs under about 1 cubic yard (45 bags of 80 lb).
• Planning a patio surface instead? Compare the cost and effort with the paver calculator, which also works out base gravel and bedding sand.

When to call a professional

Patios, walkways and post footings are well within DIY range. Structural work is different: foundations, load-bearing footings, retaining walls and driveways that carry heavy loads need correct reinforcement, compaction and curing, and often a permit and inspection. Pours over about 3 cubic yards also need a crew: concrete gives you limited working time, and one person cannot place, screed and finish that much before it sets. For those jobs, use this calculator to sanity-check volumes, but follow your local code and get a contractor or engineer involved.

Features

• Slab, footing, column, stair calculations
• Results in cubic yards and cubic feet
• Pre-mixed bag quantities (40/60/80 lb)
• 10% waste factor included
• Metric conversion (cubic meters)

Frequently Asked Questions