Bags of Concrete Calculator
Find out exactly how many bags of concrete you need — for a slab, post hole or footing — and which bag size is best value.
Your project
= 0.333 ft · 101.6 mm
3× the post width is the rule of thumb.
⅓ of the post’s above-ground height.
The post displaces concrete.
Everything is calculated in your browser. Nothing you type is sent anywhere or stored.
Bags or ready-mix?
Bagging this is a big job
Mixing time
6 h 12 m
One person, 62 bags
Water & weight
46.5 gal
Water needed for the full mix
The short answer
Divide your total volume in cubic feet by the yield of one bag. An 80 lb bag yields 0.60 cu ft, a 60 lb bag 0.45 cu ft and a 40 lb bag 0.30 cu ft. So one cubic yard (27 cu ft) takes about 45 × 80 lb bags. Add 5–10% for waste and always round up — you cannot buy half a bag.
Bags per cubic yard and cubic foot
| Bag size | Yield | Per cu ft | Per cubic yard | Per pallet |
|---|---|---|---|---|
| 80 lb | 0.60 cu ft | 1.7 | 45 | 42 |
| 60 lb | 0.45 cu ft | 2.2 | 60 | 56 |
| 50 lb | 0.375 cu ft | 2.7 | 72 | 64 |
| 40 lb | 0.30 cu ft | 3.3 | 90 | 80 |
Pallet counts vary slightly between manufacturers. Check before ordering a full pallet.
How much does one bag cover?
Coverage depends entirely on thickness. The same bag that covers 3.6 sq ft at 2 inches covers only 1.2 sq ft at 6 inches.
| Thickness | 80 lb bag | 60 lb bag | 40 lb bag |
|---|---|---|---|
| 2 in | 3.6 sq ft | 2.7 sq ft | 1.8 sq ft |
| 3 in | 2.4 sq ft | 1.8 sq ft | 1.2 sq ft |
| 4 in | 1.8 sq ft | 1.35 sq ft | 0.9 sq ft |
| 5 in | 1.44 sq ft | 1.08 sq ft | 0.72 sq ft |
| 6 in | 1.2 sq ft | 0.9 sq ft | 0.6 sq ft |
Bags per fence post
The most common bagged-concrete job. These figures are per post, with a 4×4 post in the hole and no waste added.
| Hole size | Concrete | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 8″ wide × 24″ deep | 0.63 cu ft | 2 | 2 | 3 |
| 10″ wide × 24″ deep | 1.09 cu ft | 2 | 3 | 4 |
| 10″ wide × 30″ deep | 1.36 cu ft | 3 | 4 | 5 |
| 12″ wide × 30″ deep | 2.09 cu ft | 4 | 5 | 7 |
| 12″ wide × 36″ deep | 2.51 cu ft | 5 | 6 | 9 |
Most people buy 2 bags per standard fence post and accept a little surplus. Running out halfway through setting a post is the worse outcome.
How to calculate bags of concrete
Two numbers decide everything: the volume of your pour, and the yield of one bag.
- Find the volume in cubic feet. For a slab, length × width × thickness, all in feet. For a post hole, π × radius² × depth, minus the post.
- Add waste. 5–10% covers spillage and over-dug holes.
- Divide by the bag yield. 0.60 cu ft for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb.
- Round up. Always. There is no such thing as two-thirds of a bag.
Worked example
A 10 × 10 ft shed base, poured 4 inches thick, using 80 lb bags:
- Volume10 × 10 × 0.333 = 33.3 cu ft
- Plus 10% waste36.7 cu ft
- Divide by bag yield36.7 ÷ 0.60 = 61.2
- Round up62 × 80 lb bags
- Total dry weight4,960 lb
Sixty-two bags is 2.5 tons of material to move and roughly six hours of mixing for one person. At this size, ready-mix is worth pricing up.
Which bag size should you buy?
| Bag | Best for | Value | The catch |
|---|---|---|---|
| 80 lb | Slabs, footings, anything over a few bags | Lowest cost per cu ft | Heavy. Awkward to lift repeatedly. |
| 60 lb | Post holes, small pads, older backs | Slight premium | More bags to open and mix. |
| 50 lb | Fast-setting post mixes | Mid | Often only sold as fast-set. |
| 40 lb | Small repairs, single posts | Highest cost per cu ft | You will open a lot of bags. |
80 lb bags almost always win on cost per cubic foot. The calculator above compares your actual prices, so if your store is running an offer on 60 lb, it will tell you.
Water and mixing
| Bag | Water needed | Mixed volume | Mixed weight |
|---|---|---|---|
| 80 lb | ~3 quarts (2.8 L) | 0.60 cu ft | ~90 lb |
| 60 lb | ~2.25 quarts (2.1 L) | 0.45 cu ft | ~68 lb |
| 40 lb | ~1.5 quarts (1.4 L) | 0.30 cu ft | ~45 lb |
Add water gradually and stop at a workable, oatmeal-like consistency. Extra water is the single easiest way to weaken concrete — a soupy mix can lose a substantial share of its design strength.
When to stop bagging and order ready-mix
The crossover sits around half a cubic yard, or roughly 22 × 80 lb bags. Below that, bags are cheaper and simpler. Above it, three things turn against you:
- Cost per yard climbsBagged concrete typically costs well over double ready-mix per cubic yard.
- Labour becomes the real priceFifty bags is most of a day for one person, before any finishing work.
- Cold jointsA slab mixed batch by batch never sets as one monolithic pour.
Watch the supplier’s short-load fee though — below three or four yards it can wipe out the saving. Price both.
Common mistakes
- Using volume instead of yieldAn 80 lb bag does not make 80 lb of concrete-worth of volume. It yields 0.60 cu ft. Always divide by yield.
- Forgetting the post displaces concreteA 4×4 in a 10″ hole takes up real volume. Ignore it and you over-buy on every single post.
- Buying the exact countRound up, then add one. A spare bag costs a few dollars; a second trip to the store costs an afternoon.
- Adding water to make it easierThe most common cause of weak DIY concrete. Mix to a workable consistency and no wetter.
- Confusing concrete mix with mortar or sand mixOnly concrete mix contains aggregate. The others are not substitutes for structural work.
- Bagging a job that wants ready-mixAbove half a yard you are paying double and working ten times as hard.
What this calculator assumes
Adjust your result if your project differs from these.
- Bag yields of 0.60, 0.45, 0.375 and 0.30 cu ft for 80, 60, 50 and 40 lb bags
- Standard concrete mix, not fast-setting or high-strength products
- Post holes treated as true cylinders (real holes are usually slightly wider at the top)
- Actual lumber dimensions for post displacement (a 4×4 is 3.5″)
- Water of roughly 3 quarts per 80 lb bag
- Mixing at about 6 minutes per 80 lb bag, per person
- Pallet counts of 42, 56, 64 and 80 bags — verify with your supplier
- Prices are whatever you enter; they are not live market rates
Common questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet, so you need about 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags.
How many bags of concrete for a fence post?
A 4×4 post in a 10″ wide, 24″ deep hole needs about 2 × 80 lb bags. Most people buy 2 per post and accept a little surplus.
How much does an 80 lb bag cover?
It yields about 0.60 cu ft. At 4 inches thick that is roughly 1.8 sq ft; at 2 inches, about 3.6 sq ft.
How much water does a bag of concrete need?
About 3 quarts (2.8 L) for an 80 lb bag, 2.25 quarts for 60 lb and 1.5 quarts for 40 lb. Add it gradually — too much water badly weakens the mix.
Is it cheaper to buy bags or ready-mix?
Bags are cheaper below about half a cubic yard. Above that, ready-mix wins on cost per yard and saves hours of labour. Check the short-load fee first on small orders.
Which bag size is the best value?
80 lb bags almost always give the lowest cost per cubic foot. 60 lb bags cost a little more per volume but are much easier to lift — which matters at bag 40 of 60.
How many 80 lb bags are on a pallet?
Typically 42. A 60 lb pallet holds about 56 bags and a 40 lb pallet about 80. Counts vary by manufacturer.
How long does it take to mix a bag?
Roughly 5–6 minutes per 80 lb bag for one person, including placing. Fifty bags is most of a working day.
Do I need to add gravel or sand to bagged concrete?
No. Concrete mix already contains cement, sand and aggregate. Just add water. Mortar mix and sand mix are different products and are not substitutes.
Can I dry-pour concrete into a post hole?
Fast-setting post mix is designed for it — pour it in dry and add water. Standard concrete mix is meant to be mixed first, though many people dry-set posts successfully in damp ground.
How many bags for a 10×10 slab?
At 4 inches thick that is 33.3 cu ft, or about 56 × 80 lb bags before waste. Add 10% and you are buying 62.
How much does a bag weigh once mixed?
An 80 lb bag makes about 0.60 cu ft of concrete weighing roughly 90 lb once water is added.
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materialcalculate.com/bags-of-concrete-calculator/
Project
| Type | — |
| Dimensions | — |
| Waste allowance | — |
Volume
| Cubic feet (incl. waste) | — |
| Cubic yards | — |
| Cubic metres | — |
Shopping list
| 80 lb bags | — |
| 60 lb bags | — |
| 40 lb bags | — |
| Water needed | — |
| Dry weight to move | — |
| Estimated cost | — |
Notes
Estimate generated by materialcalculate.com. Assumes standard concrete mix with yields of 0.60, 0.45, 0.375 and 0.30 cu ft for 80, 60, 50 and 40 lb bags. Confirm quantities with your supplier before buying. Not a substitute for engineering advice.