Calculate concrete slab volume and how many bags of mix you need, in either US or metric units.
Enter your slab dimensions and click Calculate to see how much concrete you need
This free concrete calculator works out the volume of a concrete slab — in cubic feet, cubic yards, or cubic meters — and how many bags of pre-mixed concrete you need to pour it, based on your slab's length, width, and thickness. It's built for DIYers and small contractors pouring patios, walkways, footings, and small slabs who want an accurate material estimate before buying bags, and it supports both US (feet/inches) and Metric (meters/centimeters) units in one tool.
The calculator multiplies slab length by width by thickness (after converting thickness from inches or centimeters into feet or meters) to get the total volume. In US mode, it converts cubic feet to cubic yards by dividing by 27, since ready-mix concrete is typically ordered by the cubic yard. It then divides the total volume by the yield of your selected bag size — for example, a 60 lb bag yields about 0.45 cubic feet, or a 25 kg bag yields about 0.0125 cubic meters — and rounds up to the number of whole bags needed.
This concrete slab calculator is useful for homeowners planning a weekend patio or walkway pour, contractors putting together a quick material estimate for a client quote, and anyone comparing the cost and effort of bagged concrete mix versus a ready-mix delivery truck. It's a planning tool, not a substitute for a structural engineer's calculations on load-bearing work like foundations or retaining walls.
Concrete volume scales directly with thickness, so a small miscalculation in slab depth can significantly change how many bags you need — running short partway through a pour can create a visible seam or "cold joint" where the concrete doesn't fully bond. Getting an accurate bag count up front helps you buy the right amount and pour the whole slab in one continuous session.
The exact formula this calculator uses to turn slab dimensions into volume and bags needed
The calculator always works in a single consistent unit behind the scenes: it converts your thickness entry into feet (US mode) or meters (Metric mode) before multiplying by length and width, so a slab measured in inches or centimeters is never accidentally treated as feet or meters.
Length × Width × Thickness gives raw volume. Thickness is always converted to the same unit as length and width first — inches ÷ 12 for feet, or centimeters ÷ 100 for meters.
US mode divides cubic feet by 27 to get cubic yards, since ready-mix concrete is ordered by the cubic yard. Metric mode multiplies cubic meters by 1,000 to show liters alongside cubic meters.
Total volume is divided by the selected bag's yield, then rounded up (never down) — since a partial bag still needs to be purchased as a whole bag.
From entering your slab dimensions to reading your bag count
Select US (ft / in) or Metric (m / cm) depending on how you'll be measuring your slab.
Type your slab's length and width into the corresponding fields, in feet for US mode or meters for Metric mode.
Enter your slab's thickness in inches (US mode) or centimeters (Metric mode) — this is the depth of the pour, not the surface area.
Choose the concrete bag size you plan to buy — 40, 60, or 80 lb bags in US mode, or 25 or 40 kg bags in Metric mode.
See the total volume in two units (cubic yards and cubic feet, or cubic meters and liters) along with the number of bags needed, rounded up to the next whole bag.
Two scenarios using the calculator's own default values — one in US units, one in Metric
Suppose you're pouring a 10 ft × 10 ft patio slab at a 4-inch thickness, using 60 lb bags.
Now suppose you're pouring a smaller 3 m × 3 m pad at a 10 cm thickness, using 25 kg bags.
Explanation: In both scenarios, thickness is the number that gets converted first — into feet or meters — before it's multiplied by length and width. Switching bag sizes changes only the final division step: choosing 80 lb bags (0.60 cu ft yield) instead of 60 lb bags in Scenario 1, for example, would drop the bag count from 75 down to 56 (33.3 ÷ 0.60 = 55.5, rounded up to 56), since each bag covers more volume.
What the volume and bag count actually tell you before you head to the store
Volume (cubic yards / cubic meters): This is the number to use if you're pricing ready-mix delivery, since concrete trucks sell by the cubic yard (or cubic meter in metric regions) with a typical minimum order. If your total lands well under 1 cubic yard, bagged mix is usually the more practical route.
Volume (cubic feet / liters): This secondary figure is the more useful one for bagged concrete math, since bag yields are typically stated in cubic feet (US) or liters/cubic meters (metric), making it easy to sanity-check the bag count the calculator returns.
Bags needed: This is your shopping list number — already rounded up to a whole bag, since suppliers don't sell partial bags. It does not include the 5-10% overage most contractors add for spillage and uneven subgrade, so treat it as your baseline before padding the order.
Why ordering extra matters: Running short mid-pour is one of the most common concrete mistakes — once the surface starts to set, adding fresh concrete on top creates a visible "cold joint" seam that never fully bonds to the earlier pour. A few extra bags on hand (or a slightly larger ready-mix order) is far cheaper than a compromised slab.
This calculator provides planning estimates only. It does not replace a structural engineer's calculations for load-bearing applications like foundations, retaining walls, or structural footings — consult a licensed professional and your local building code for those projects.
Where an accurate volume and bag estimate saves you a wasted trip to the hardware store
Estimate bags needed before pouring a backyard patio slab on a weekend.
Size a walkway pour and check bag count against a standard 4-inch thickness.
Get a cubic-yard total for a driveway slab to compare against ready-mix delivery pricing.
Quickly rough out material costs for a small foundation or slab job before quoting a client.
Calculate the small volume needed for individual fence-post or gate-post footings.
Estimate the slab volume and bag count for a shed, garage, or outbuilding foundation.
Work out how much bagged mix is needed to patch or replace a single sidewalk section.
Size the footing pour that a retaining wall will sit on before construction begins.
Estimate concrete needed for individual deck-post footings or pier pads.
Rough out material needs for a small commercial pad, curb, or apron pour.
Calculate concrete for garden borders, stepping-stone bases, or landscape edging pads.
Estimate the slab volume surrounding a pool deck before ordering materials.
Compare bagged-mix cost and labor against a ready-mix truck's delivery minimum.
Students and trade-school learners can practice volume and material takeoff calculations.
Know exactly how many bags to load into the cart before you leave for the store.
What this concrete calculator does well, and where it can't replace professional judgment
Standard bag yields used by this calculator, and how many bags each size takes to fill a cubic yard or cubic meter
| Bag Size | Yield per Bag | Bags per Cubic Yard / Cubic Meter |
|---|---|---|
| 40 lb bag | 0.30 cu ft | 90 bags per cu yd |
| 60 lb bag | 0.45 cu ft | 60 bags per cu yd |
| 80 lb bag | 0.60 cu ft | 45 bags per cu yd |
| 25 kg bag | 0.0125 m³ | 80 bags per m³ |
| 40 kg bag | 0.02 m³ | 50 bags per m³ |
| Slab Area | Volume (cu yd) | 60 lb Bags Needed |
|---|---|---|
| 8 × 8 ft (64 sq ft) | 0.79 cu yd | 48 bags |
| 10 × 10 ft (100 sq ft) | 1.23 cu yd | 75 bags |
| 12 × 12 ft (144 sq ft) | 1.78 cu yd | 107 bags |
| 20 × 20 ft (400 sq ft) | 4.94 cu yd | 297 bags |
These figures assume a constant 4-inch thickness and 60 lb bags with a 0.45 cu ft yield. Doubling the thickness roughly doubles both the volume and bag count — always re-run your own numbers above for your exact dimensions and bag size.
Summary: This concrete calculator turns your slab's length, width, and thickness into a volume in cubic yards/feet or cubic meters/liters, plus the number of bags needed for your chosen bag size — a fast, free planning tool for patios, walkways, footings, and small slabs. Pair it with the Tile / Flooring Calculator or Paint Calculator to plan the rest of the project once the slab is poured.
Common questions about estimating concrete volume and bags
Official industry guidance to complement this calculator — not a substitute for a licensed contractor or structural engineer
Other home & project tools