Enter the length, width and thickness of the pour, then allow for an uneven base and a little handling loss. We’ll show cubic yards and common bag-size estimates.
These bag yields are planning assumptions. Check the exact mix label, and get project-specific advice for structural work, foundations or any slab with design requirements.
How to use this calculator
- Measure the pour and set the thickness honestly — measure the excavation, including any low spots.
- Add at least 10 percent for an uneven base and spillage; concrete cannot be stretched on the day.
- Compare bag sizes by cost per cubic foot, and be honest about how many heavy bags you can mix in a session.
- If the count passes roughly 80 to 100 bags, price a ready-mix delivery — it is often cheaper and far less brutal.
How concrete quantities are calculated
Volume in cubic feet = length x width x (thickness in inches / 12), increased by your waste percentage. Cubic yards = cubic feet / 27. Bags = volume / yield per bag: 0.60 cu ft for 80 lb, 0.45 for 60 lb and 0.30 for 40 lb bags.
A 10 ft x 10 ft slab at 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards before waste. In 80 lb bags that is 56 bags; with the 10 percent allowance the working figure is about 62 bags — firmly in 'consider ready-mix' territory for most people.
A few things worth checking
- Measuring thickness at the edges only, while the middle of the excavation is an inch deeper — that extra inch on a big slab is another dozen bags.
- Planning to mix a huge count of bags solo. Concrete waits for no one; a slab must be placed and finished while it is workable.
- Forgetting water, tools and curing in the plan: you need mixing water on site, a way to level (screed) and a way to keep the slab damp while it cures.
Your questions, answered
How many 80 lb bags are in a cubic yard?
Using this tool’s assumed mixed yield of 0.60 cu ft per 80 lb bag, 27 cu ft needs 45 bags before waste. The other fixed yields are 0.45 cu ft for a 60 lb bag and 0.30 cu ft for a 40 lb bag. Confirm the actual mixed yield printed on your chosen product.
When is ready-mix cheaper than bags?
This tool does not calculate cost. Use the volume result and a checked bag yield to compare supplier quotes, delivery or minimum-load charges, mixing arrangements and the work required for your project.
Does the tool choose a slab thickness?
No. It uses the thickness you enter to estimate volume and bags. The 4-inch starting value is an input assumption; it does not approve a slab design, reinforcement, base or loading.
Does it tell me when concrete is ready for traffic?
No. It estimates material quantities only. Placement, curing and the time before use depend on the chosen mix and site conditions. Follow the actual product instructions and project requirements.
Concrete Bags: Plan the Volume Before You Buy
Use the yield on the bag, leave room for an uneven base and make a plan for the pour.