Concrete bag calculator
Slabs, footings, post holes. Tell it your dimensions, pick your bag size, get an exact count with waste factor included.
Quick answer
How many bags of concrete do I need?
Intro
When to use this calculator
An 80 lb bag of Quikrete makes 0.6 cubic feet of mixed concrete. Memorize that number and most concrete projects become arithmetic. This calculator handles the rest.
Pick the shape (slab, footing, post hole, or column), enter the dimensions, and the calculator returns the bag count for whichever bag size you're buying. Built-in waste factor is 10% for slabs, 15% for footings, and 20% for post holes. Those are the numbers that have actually saved us trips back to the store.
The calculator assumes you're using standard 4,000 PSI Quikrete or Sakrete from Home Depot or Lowe's. If you're using a specialty mix (fast-setting, high-strength, fiber-reinforced), the bag yield can drop 5 to 10%. Buy one extra bag to be safe.
How to use it
How to use this calculator
- Pick the shape that matches your project. Slab for patios and pads, footing for fence and deck supports, post hole for fence posts, column for sonotube pours.
- Enter dimensions in feet and inches. The calculator converts them automatically.
- Pick your bag size. 80 lb is the most common, 60 lb is easier to lift, 40 lb is for small repairs.
- The result is total bags needed, with waste factor already added.
- For complex shapes, calculate each section separately and add them up.
Default 10%. Bump to 15% for irregular pours or first-timers.
Pocket conversion table
Formula
Concrete bag math
bags = ceil( (L × W × T_ft × (1 + waste%)) / yield_per_bag ) Where: L, W in feet · T_ft = thickness_inches / 12 yield = 0.30 ft³ (40 lb) · 0.45 ft³ (60 lb) · 0.60 ft³ (80 lb) waste = 0.10 standard, 0.15 for irregular pours
π × r² × depth, then subtract the post's own volume. Always round bag counts up, never down.Worked example
10 × 10 ft slab at 4 inches thick
Backyard pad for a shed. Standard 80 lb Quikrete bags.
- 1. Volume = 10 × 10 × (4 / 12)33.33 ft³
- 2. With 10% waste36.67 ft³
- 3. Bags = 36.67 / 0.6061.1
- 4. Round up62 bags
→ Buy 62 × 80 lb bags (or 1.4 cubic yards of ready-mix)
Reading the output
Understanding your result
The big number is total bags, with waste factor already added. Always round up. The smaller numbers (cubic feet, cubic yards, adjusted volume) are there so you can sanity-check the math or convert to ready-mix delivery if the truck makes more sense.
What the bag count assumes about waste: 10% for slabs and pads, 15% for footings (more soil contact and variation), 20% for post holes (the hole walls absorb concrete). Use the waste slider to see the difference.
When to round up vs round down: always round up. An extra $7.50 bag is cheap insurance. The Saturday afternoon trip back to Home Depot when you're a half-bag short costs more in time and gas than wasting a full bag.
When to bump the waste factor higher: first-time pour, irregular or curved shapes, cold-weather pour where you can't pause to mix more, or any pour with thick edges or thickened slab perimeters. Bump from 10% to 15 or 20% in those cases.
If the result crosses about 1 cubic yard (45 bags of 80 lb), check the ready-mix recommendation. Above that volume, a truck usually beats bags on price and labor.
Common mistakes (and how to avoid them)
✗ Treating bag weight as bag volume.
Fix: An 80 lb bag is 0.6 cubic feet, not 1 cubic foot. The math collapses if you mix these up. Memorize 0.6 ft³ for 80 lb and the rest are easy multiples.
✗ Using rectangular formulas for round holes.
Fix: Post holes are cylinders. The volume is π × r² × h, not d × d × h. The wrong formula overestimates by about 21% on a 12-inch hole.
✗ Forgetting waste factor.
Fix: Concrete vanishes during pours: spillage, soil voids, sticky bucket sides. Always add 10 to 20% extra depending on project type.
✗ Mixing too much water in.
Fix: Adding water to make stirring easier drops the cured PSI. A 4,000 PSI mix with 50% extra water cures to about 2,500 PSI. Mix at the bag's specified water ratio even if it's harder.
✗ Pouring on a freezing forecast.
Fix: Concrete poured below 40°F can lose 50% of its strength permanently. Check the 48-hour forecast and use a cold-weather mix below 50°F.
Frequently asked questions
How many bags of concrete do I need for a 4-inch slab?
Can I mix Quikrete with regular sand and gravel?
How long after mixing do I have to pour concrete?
Can I pour concrete in the rain?
What's the difference between Quikrete and Sakrete?
Tools you may need
Basic mixing kit. Most homeowners already have a shovel and level, borrow or rent the rest before buying.
- Wheelbarrow. For mixing and moving, a 6 cu ft contractor model handles 2–3 bags at a time.
- Mixing tub or mortar pan. Cleaner than a wheelbarrow for small batches; easier on your back.
- Mason hoe. The hoe with two holes, moves wet mix far better than a garden hoe.
- Round-point shovel. For scooping mix and screeding small slabs.
- 4 ft level. Check the form before you pour. Fixing a sloped slab after the fact is not really fixable.
Related guides
Concrete PSI ratings explained
3,000 vs 4,000 vs 5,000 PSI, and which one your project actually needs.
OpenConcrete curing time by temperature
When you can walk on it, drive on it, and load it. Cold weather changes everything.
Open6 concrete bag math mistakes
The arithmetic errors that send people back to Home Depot mid-pour.
OpenBags of concrete per fence post
Frost-line aware bag math for 4×4, 6×6, and round posts.
OpenWhere to buy your materials
We've linked to common products at Home Depot and Lowe's below. These are affiliate links, meaning we earn a small commission if you buy through them, at no cost to you. We've used these specific products on projects ourselves. If your local independent yard has a better price, take it.
Concrete mix
×62Quikrete 80 lb bags, the standard SKU at both retailers.
Local stock and per-bag price swing week to week. Check both before buying a pallet.
Affiliate links, same price for you, helps keep the calculators free.
Cost estimate
Rough material-only ranges based on typical US retail prices. Doesn't include rebar, forms, or delivery.
Above ~1 cubic yard, ready-mix from a truck is usually cheaper than bags. See the section below.
Save money, past 1 cubic yard
You're past 1 cubic yard, ready-mix is cheaper
A ready-mix truck saves you a day of mixing and usually costs less than 62 bags. Get 3 free quotes from local suppliers, no obligation.
Free service. Takes 30 seconds. No pushy sales calls.
Local pricing · Miami-Dade & Broward
Need delivery for this pour?
At 1.23 yd³, a ready-mix truck almost always beats 62 bags on price, time, and pour consistency, once you're past about 1 yard, the short-load fee stops eating the savings. We're starting MaterialMath's local supplier coverage in Miami-Dade and Broward first, send your numbers and we'll help you compare 2–3 yard quotes.
- Compare price per yard, short-load fee, and minimum order across plants.
- Confirm mix design (3000 vs 4000 PSI), slump, and any fiber or air-entrainment add-ons you need.
- Confirm truck access, pour window, and how long you have to unload before standby fees kick in.
Outside South Florida? Search "ready-mix concrete near me" and ask the same questions, more regions coming soon.
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