Deck Block Calculator
A floating deck sits on precast concrete deck blocks instead of dug footings. The count comes from two spacings: how far apart the rows of blocks are (the joist span between supports) and how far apart the blocks are along each row. This lays the blocks out, checks the load on each against what the soil under it can carry, and works out the compacted gravel pads the blocks should sit on.
How the deck block count works
A floating deck is a grid. The joists run across rows of blocks, so the distance between rows is the joist span, and the blocks along each row hold up the beam or the joist that sits in their slots. Count the rows across the width, count the blocks along the length, and multiply: rows = width ÷ row spacing, rounded up, plus one; blocks per row = length ÷ block spacing, rounded up, plus one.
Worked example: a 12 × 16 ft floating deck
| Step | Working | Result |
|---|---|---|
| Rows across 12 ft at ≤ 6 ft | 12 ÷ 6 = 2, + 1 | 3 rows, 6 ft apart |
| Blocks along 16 ft at ≤ 4 ft | 16 ÷ 4 = 4, + 1 | 5 per row, 4 ft apart |
| Total | 3 × 5 | 15 blocks |
| Load on an interior block | 6 ft × 4 ft × 50 psf | 1,200 lb |
| Soil under an 11 in block | 0.84 sq ft × 1,500 psf | 1,260 lb — OK |
The 50 psf is the 40 psf live load plus 10 psf dead load that the IRC and the AWC's DCA 6 use for residential decks, and 1,500 psf is the IRC Table R401.4.1 presumptive bearing value for clay or unknown soil. Build a 16 ft wide deck with rows 8 ft apart and the interior load climbs to 8 × 4 × 50 = 1,600 lb, more than a small block can spread on average soil — which is why the calculator flags it.
Set every block on a gravel pad
A block on bare topsoil settles unevenly. Strip the sod, dig each spot about 4 in down and 24 in square, fill it with crushed stone and tamp it level. Fifteen pads come to about 20 cubic feet of gravel, roughly 0.74 cubic yards or 40 bags. Angular #57 stone locks together and drains; round pea gravel rolls and does not. Size a bulk order with the #57 stone calculator, and if you are covering the ground under the whole deck as well, the gravel under deck calculator works out the stone and landscape fabric in one go.
When deck blocks are the wrong choice
- Attached decks. A block moves with frost; a ledger bolted to the house does not. Attached decks need footings below frost depth.
- Elevated decks. Blocks resist almost no sideways force. Keep a floating deck low — under about 30 in, and lower is better.
- Heavy loads. A hot tub or a masonry fire feature needs its own footing or pad.
- Permitted decks. If the deck needs a permit, the inspector will want footings; check the deck footing calculator.
Pricing the rest of the deck? The floating deck cost calculator totals the boards, framing and labour, and the deck joist calculator counts the joists that run across the rows.
Get the Smart Decking Calculator app for Android
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Frequently asked questions
- How many deck blocks do I need for a 12x16 deck?
- Fifteen, laid out as three rows of five. With rows no more than 6 ft apart across the 12 ft width you need three rows, and with blocks no more than 4 ft apart along the 16 ft length each row needs five. Each interior block then carries about 1,200 lb, just under the 1,260 lb an 11 in block can put on 1,500 psf soil.
- How far apart should deck blocks be?
- About 4 ft apart along each row and 6 ft or less between rows is the common layout for a floating deck. The real limit is load: each interior block carries the deck area around it at about 50 lb per square foot, and a small block on average soil can only spread roughly 1,200 lb. Closer spacing or bigger blocks fix an overloaded layout.
- Do deck blocks need gravel under them?
- They should sit on a compacted gravel pad rather than bare soil. Strip the topsoil, dig down about 4 in, fill with crushed stone such as #57 and tamp it level so the block cannot rock or settle into mud. A 24 in square pad 4 in deep is about 1.3 cubic feet of gravel, or roughly three 0.5 cubic foot bags.
- Are deck blocks up to code?
- Usually only for a low, freestanding deck that the code exempts from a permit, often under 30 in above grade and under 200 sq ft. Deck blocks sit above the frost line, so the ground can lift them in winter. Anything attached to the house, elevated or permitted needs footings dug below frost depth, which your building department sets.
- Can I put a beam directly on deck blocks?
- Yes. Most blocks have a slot for a 2x joist or beam on edge and a pocket for a 4x4 post. Setting beams in the slots and running joists across them keeps the deck lowest. Use short posts only where the ground falls away, and keep them short, because a block does not hold a post against sideways movement the way a buried footing does.
