Concrete & Decking Estimator
Work out how much concrete you need for a slab — in cubic yards, cubic metres and bags — or how many boards, joists, posts and screws a deck needs. Enter your measurements below and the calculator does the arithmetic for you. No prices are built in: any cost shown comes only from a price you enter yourself.
Concrete & Decking Calculator
Bag yields are approximate and vary by product — check the yield printed on the bag label and change these figures to match.
Boards run along the deck length. Joists run across the deck width. The post estimate assumes two beam lines with posts at the spacing above — check your deck design and local code before building.
Your estimate
How concrete is calculated
Concrete is sold by volume. First work out the volume of the slab, then convert it into the units your supplier uses.
- Volume: length × width × depth. Keep all three measurements in the same unit first — for example, convert depth in inches to feet by dividing by 12.
- Cubic feet to cubic yards: divide cubic feet by 27 (1 cubic yard = 27 cubic feet).
- Cubic feet to cubic metres: multiply cubic feet by 0.0283168.
- Bags: divide the volume in cubic feet by the yield of one bag. The calculator defaults to about 0.6 cu ft per 80 lb bag and about 0.45 cu ft per 60 lb bag — both approximate, so check the bag label.
- Waste: add about 10% for spillage, uneven ground and over-excavation. Order a little extra rather than running short mid-pour.
How decking is calculated
- Deck area: length × width.
- Deck boards: divide the deck width by one board width plus one gap, then round up. Boards are assumed to run along the deck length, so each board is as long as the deck.
- Joists: divide the deck length by the joist spacing, then add one joist for the starting edge. The default spacing is 16 in (400 mm) on centre — change it to match your design.
- Posts: the calculator assumes two beam lines, with posts spaced along the deck length (default 8 ft apart). The exact number depends on your deck design, footings and local code.
- Screws: about 2 screws per board at every joist it crosses. This is an approximate estimate — fixing patterns vary, so confirm with your board and fixing supplier.
Worked examples
Example 1 — Concrete slab: 10 ft × 12 ft, 4 in thick
- Convert depth: 4 in ÷ 12 = 0.333 ft.
- Volume: 10 × 12 × 0.333 = 40 cubic feet.
- Cubic yards: 40 ÷ 27 = 1.48 cubic yards.
- Cubic metres: 40 × 0.0283168 = 1.13 cubic metres.
- 80 lb bags: 40 ÷ 0.6 = 66.7, rounded up to 67 bags. With a 10% waste allowance: 44 cu ft ÷ 0.6 = 73.3, rounded up to 74 bags.
- 60 lb bags: 40 ÷ 0.45 = 88.9, rounded up to 89 bags. With 10% waste: 44 ÷ 0.45 = 97.8, rounded up to 98 bags.
Result: a 10 ft × 12 ft slab, 4 in thick, needs about 1.48 cubic yards (1.13 m³) of concrete — 67 × 80 lb bags (or 89 × 60 lb bags), or 74 × 80 lb bags with 10% waste.
Example 2 — Deck: 12 ft wide × 16 ft long, 5.5 in boards, 0.25 in gap, joists at 16 in
- Deck area: 12 × 16 = 192 sq ft.
- Boards: deck width 12 ft = 144 in. One board plus one gap = 5.5 + 0.25 = 5.75 in. 144 ÷ 5.75 = 25.04, rounded up to 26 boards, each 16 ft long.
- Joists: deck length 16 ft = 192 in. 192 ÷ 16 = 12 spaces, plus one starting joist = 13 joists, each 12 ft long.
- Posts: posts every 8 ft along 16 ft, on two beam lines: 2 × (16 ÷ 8 + 1) = 2 × 3 = 6 posts.
- Screws: 2 screws per board per joist: 26 × 13 × 2 = 676 screws (about 700 as a rounded buying figure).
Result: a 12 ft × 16 ft deck (192 sq ft) needs about 26 deck boards (16 ft), 13 joists (12 ft), 6 posts and about 676 screws.
Concrete by slab size (4 in thick)
Volumes before any waste allowance. Bag counts use the approximate yield of 0.6 cu ft per 80 lb bag — always check the bag label.
| Slab size | Volume (cu ft) | Volume (cu yd) | 80 lb bags |
|---|---|---|---|
| 8 ft × 10 ft | 26.67 | 0.99 | 45 |
| 10 ft × 10 ft | 33.33 | 1.23 | 56 |
| 10 ft × 12 ft | 40.00 | 1.48 | 67 |
| 12 ft × 12 ft | 48.00 | 1.78 | 80 |
| 12 ft × 16 ft | 64.00 | 2.37 | 107 |
| 16 ft × 20 ft | 106.67 | 3.95 | 178 |
Decking materials list, explained
- Deck boards — the walking surface. Counted across the deck width; allow extra for cutting, mitres and any damaged boards.
- Joists — the framing boards that support the deck boards. Spacing (commonly 16 in / 400 mm on centre) affects how many you need and how stiff the deck feels.
- Posts and footings — the vertical supports that carry the deck down to the ground. Their number, size and footing depth depend on the deck height, span and local code.
- Beams — horizontal members that sit on the posts and carry the joists. Not counted by this calculator, because beam size and layout depend on the design.
- Screws / fixings — about 2 per board per joist crossing, as an approximate estimate. Use fixings rated for outdoor and decking use.
- Extras — joist hangers, post bases, flashing and edging boards, as your design requires.
What to buy — shopping list
Use the calculator above, then take the results as your list:
- For concrete: ready-mix concrete in cubic yards, or the bag count shown (80 lb or 60 lb bags), plus the 10% waste allowance if you ticked it. Check the bag yield on the label before you buy.
- For a deck: the deck boards, joists and posts shown, plus screws (rounded up to the nearest box size), and any hangers, post bases and flashing your design needs.
- Confirm quantities with your supplier — they can check your measurements against the product sizes they actually stock.
Frequently asked questions
How much concrete do I need for a 10 ft × 10 ft slab, 4 inches thick?
10 × 10 × (4 ÷ 12) = 33.33 cubic feet, which is 33.33 ÷ 27 = 1.23 cubic yards. At about 0.6 cubic feet per 80 lb bag, that is 33.33 ÷ 0.6 = 56 bags before any waste allowance. Always check the yield printed on the bag you buy.
How many bags of concrete are in a cubic yard?
One cubic yard is 27 cubic feet. At an approximate yield of 0.6 cubic feet per 80 lb bag, one cubic yard needs 27 ÷ 0.6 = 45 bags. At about 0.45 cubic feet per 60 lb bag, it needs 27 ÷ 0.45 = 60 bags. Bag yields vary by product, so check the bag label.
How thick should a concrete slab be?
Slab thickness depends on what the slab will carry, the ground underneath it, and local building rules. There is no one thickness that suits every job. Check your local code and ask a qualified professional before deciding on depth, reinforcement, footings or loads.
How many deck boards do I need?
Divide the deck width by the width of one board plus its gap, then round up. For example, a 12 ft (144 in) wide deck with 5.5 in boards and a 0.25 in gap needs 144 ÷ 5.75 = 25.04, rounded up to 26 boards, each the length of the deck.
Should I add extra concrete or decking for waste?
Yes — a 10% waste allowance is a common planning figure for concrete and decking, to cover spillage, uneven ground, cutting and offcuts. The calculator above has a 10% waste option. For unusual shapes or a lot of cutting, ask your supplier or contractor whether more is sensible.
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Disclaimer: These are planning estimates only, not structural or building-code advice. Concrete and decking work, footings, spans and loads need a qualified professional and a check against your local building code before you build. Measurements, bag yields and product sizes vary — always confirm quantities with your supplier and the product label.