Concrete Bag Calculator: How Many 80lb, 60lb & 50lb Bags for Slab
Buying too few bags means a mid-pour trip to the hardware store; buying too many wastes money and leftover mix that hardens in the bag. This calculator converts your slab dimensions into cubic feet and cubic yards, then rounds up to the nearest whole bag for 80lb, 60lb or 50lb mixes, with an optional cost estimate.
You'll need 62 bags of 80 lb concrete for a 36.67 cu ft pour, about $403.
| Metric | Value |
|---|---|
| Shape | Rectangular |
| Area (sq ft) | 100.00 |
| Depth (ft) | 0.333 |
| Base volume (cu ft) | 33.33 |
| With waste allowance (cu ft) | 36.67 |
| Cubic yards | 1.358 |
| Bags required | 62 |
| Estimated cost | $403 |
- 1.
Area = Length × Width (rectangular) or π × (Diameter/2)² (round) - 2.
CubicFeet = Area × (Depth_in / 12) × (1 + Waste%/100) - 3.
CubicYards = CubicFeet / 27 - 4.
Bags = ceil(CubicFeet / CuFtPerBag)
How to Use This Calculator
1. Enter your details
Use the labelled fields and units that match your situation.
2. Calculate instantly
The result, visual comparison, and working update as values change.
3. Review your result
Check assumptions, then copy, print, or share the calculation.
Formula & step-by-step maths
- CuFtPerBag
- Yield per bag: 0.60 (80lb), 0.45 (60lb), 0.375 (50lb)
- Waste%
- Extra percentage added for spillage and subgrade unevenness
Rectangular vs round slabs
For rectangular slabs, volume is simply length × width × depth. For round slabs like a pier footing or fire pit base, the tool uses π × radius² × depth, where radius is half the diameter you enter. Always measure diameter, not radius, in the width field for round shapes.
Why bag yield varies by weight
An 80lb bag yields about 0.60 cubic feet of mixed concrete, a 60lb bag about 0.45 cubic feet, and a 50lb bag about 0.375 cubic feet — the yield per pound is nearly constant across sizes, so smaller bags simply mean more bags to carry and mix, not better value.
Why waste allowance matters
Uneven subgrade, spillage during mixing, and over-excavation typically consume 5-10% more material than the theoretical volume. Slabs with irregular edges or hand-dug footings should use 10-15% waste allowance; simple rectangular forms can often use 5%.
Bags vs ready-mix trucks
Once a pour exceeds roughly 1-1.5 cubic yards (about 45-60 bags of 80lb mix), a ready-mix delivery usually becomes more cost-effective and far less physical labor than hand-mixing dozens of bags, so use the cubic yard figure to decide which route makes sense.
Bags needed per cubic yard by bag size
| Bag size | Cu ft per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 | 45 |
| 60 lb | 0.45 | 60 |
| 50 lb | 0.375 | 72 |
| 40 lb | 0.30 | 90 |
Frequently Asked Questions
Three worked examples
Same engine, three different starting points — useful if you want to see how sensitive the answer is before you type your own numbers in.
Example 1: length 7.8 ft, slab shape "Rectangular"
On the lower / more conservative end. You'll need 48 bags of 80 lb concrete for a 28.60 cu ft pour, about $312.
Example 2: length 10 ft, slab shape "Rectangular"
A typical middle-of-the-road setup. You'll need 62 bags of 80 lb concrete for a 36.67 cu ft pour, about $403.
Example 3: length 13 ft, slab shape "Round"
On the higher / more demanding end. You'll need 48 bags of 80 lb concrete for a 28.80 cu ft pour, about $312.
Quick answers about the Concrete Bags
What exactly does the Concrete Bags work out?
Buying too few bags means a mid-pour trip to the hardware store; buying too many wastes money and leftover mix that hardens in the bag. You enter slab shape, length, width / diameter and depth (plus 3 more optional details) and the result panel updates straight away, so you can compare two or three versions of the same question in a few seconds.
What do I need before I start?
Only 6 fields: slab shape, length, width / diameter, depth, bag size and waste allowance. price per bag is optional — leaving it blank simply switches the tool to the simpler estimate.
How is it calculated — rectangular vs round slabs?
For rectangular slabs, volume is simply length × width × depth. The same maths runs inside this page, so hand-checking the result on paper gives you the identical figure.
Why do two calculators give me different answers for concrete Bags?
An 80lb bag yields about 0. Different sites pick different assumptions, so always check which method a calculator states before you trust the gap between two numbers.
What does the "Bags needed per cubic yard by bag size" table on this page tell me?
It is the reference range this tool works against — 4 rows from "80 lb" (0.60) up to "40 lb" (0.30). Use it to sanity-check whether the number you just calculated sits where you expected it to.
Which slab shape should I pick?
The dropdown offers 2 choices — Rectangular and Round. Pick the one that matches your real situation rather than the one you would like to be true; slab shape usually moves the final figure more than any other single input, so it is worth running it twice with the option above and below your guess.
Do I have to press a button or reload the page to see the result?
No. Concrete Bags runs completely inside your browser, so the moment you change a value the cards recalculate — there is no submit step, no page reload and no waiting for a server round trip. That also means it keeps working on a weak or intermittent mobile connection.
Is it free, and do you keep what I type?
It is free with no sign-up, no app install and no usage limit. Nothing you enter into Concrete Bags leaves your device — the calculation is JavaScript running locally, so there is no upload of your figures to DrHint or anyone else.
Can I use it on a phone?
Yes — the layout stacks to a single column on small screens and the number fields open the numeric keypad on both Android and iOS. Many people bookmark this page or add it to their home screen and re-open it whenever the question comes up.
Anything to be careful about with the result?
A 10x10 ft slab at 4 inches deep is about 33. Treat the output as a well-grounded estimate for planning, not as a professional, legal or medical decision on its own.
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