Concrete Bag Calculator — How Many Bags of Concrete
Find how many 40, 50, 60, 80 or 90 lb (or 20–40 kg) bags of premixed concrete you need for a slab, post holes or a known volume, with weight and cost.
Bagged premix is the easy way to pour small jobs such as fence posts, deck piers, shed pads and steps. This calculator works out how many bags to buy from the size of a slab, the number and size of post holes, or a volume you already know, using a listed yield or your bag-label override. It also compares every common bag size so you can weigh fewer heavy bags against more light ones.
How to use this tool
- Choose US or metric units.
- Pick what you are filling: a slab or pad, post holes, or a known volume.
- Enter the dimensions — for post holes, the hole diameter and depth, the number of holes and the post size to subtract.
- Choose the bag size, or type in the yield from your bag label.
- Set the waste allowance and, optionally, the price per bag.
Formula
Bags = volume × (1 + waste %) ÷ yield per bag, rounded up
The yield is the volume of finished concrete one bag makes: about 0.30 ft³ for 40 lb, 0.375 ft³ for 50 lb, 0.45 ft³ for 60 lb, 0.60 ft³ for 80 lb and 0.675 ft³ for 90 lb. Metric bag yields here are scaled from the same figures, about 0.47 litres per kg. A post hole holds π × (D ÷ 2)² × depth, minus the square post.
Example
A 4 × 4 ft pad 4 in thick is 5.33 ft³. At 0.60 ft³ per 80 lb bag that is 8.9 bags, so 9 bags with no waste, or 10 with 10% extra. Four fence-post holes 10 in across and 36 in deep, holding 4 × 4 posts (3.5 in actual), need 5.52 ft³ before waste: 15 bags of 50 lb mix.
Frequently asked questions
How many 80 lb bags are in a yard of concrete?
45. A cubic yard is 27 ft³ and each 80 lb bag yields about 0.60 ft³. The same yard takes 60 bags of 60 lb or 90 bags of 40 lb.
How many bags do I need for one fence post?
A 10 in hole 3 ft deep around a 4 × 4 post holds about 1.38 ft³ — 3 bags of 80 lb or 4 bags of 50 lb.
Do bigger bags make stronger concrete?
No. Strength depends on the product — standard, high-strength or fast-setting — not on the bag size. Bigger bags mean fewer trips; 40 or 50 lb bags are easier to lift and mix.
Should I buy extra bags?
The entered waste percentage is already included in the count. Decide any further contingency from actual dimensions and handling losses. Follow the product’s storage instructions and shelf-life limits for unused bags.
Sources
Last updated: 2026-10-30