Joist Cut Calculator
Plan deck and floor joist cuts from standard timber lengths, work out exactly what stock to buy, and reduce waste across rim boards, interior joists and blocking.
Why Joist Frames Benefit from a Cut List Optimiser
A deck or floor frame is built almost entirely from repeated lengths: typically one span dimension repeated many times, plus a perimeter of rim boards. That regularity makes the frame seem simple to plan, but stock length choice still has a big effect on how much timber you need to buy.
The optimiser works out whether to use 2.4m, 3.0m, 3.6m or 4.8m lengths for each cut, and how many of each, to cover all your joists, rim boards and blocking with the least waste. It also includes blade kerf, so the plan reflects real workshop cuts rather than idealised numbers.
- A standard deck frame involves 20 or more cuts across rim boards, joists and blocking.
- Repeated joist lengths make stock choice critical: a small mismatch costs you extra lengths.
- Blocking lengths are easy to underestimate and benefit from being planned alongside the joists.
- Kerf on a full deck frame can account for 60mm or more of lost material if left out.
Worked Example: 3.6m × 2.4m Deck Frame
A garden deck with a 3.6m × 2.4m footprint, joists spanning the 2.4m dimension at 400mm centres, using 4×2 (100mm × 50mm) timber throughout.
How the Parts Are Calculated
- Long rim boards: run the full 3600mm length of the deck: 2 × 3600mm
- Short rim boards: span the 2.4m width between the long rim boards: 2400mm − 50mm − 50mm = 2 × 2300mm
- Interior joist count: 3600mm ÷ 400mm spacing = 9 bays, minus the 2 rim board positions = 8 interior joists at 2300mm
- Blocking length: 400mm joist spacing − 50mm joist thickness = 350mm
- Blocking count: one row at mid-span across 8 bays = 8 blocks
Cut List: 4×2 (100mm × 50mm)
| Label | Length (mm) | Qty | Notes |
|---|---|---|---|
| Long rim board | 3600 | 2 | Long sides of deck perimeter |
| Short rim board | 2300 | 2 | Short ends between long rim boards |
| Interior joist | 2300 | 8 | At 400mm centres across the 3.6m span |
| Blocking | 350 | 8 | Mid-span bridging between joists |
Choosing the Right Stock Length for 2300mm Joists
There are 10 members at 2300mm in this example (2 short rim boards + 8 interior joists). Stock length choice makes a significant difference:
- 2.4m lengths: One joist per length (2300mm + 100mm waste minus kerf). You need 10 lengths.
- 3.6m lengths: One joist per length (2300mm + 1300mm waste). More wasteful than 2.4m for this span.
- 4.8m lengths: Two joists per length (2 × 2300mm = 4600mm plus 3mm kerf = 4603mm, fits in 4.8m). You need just 5 lengths, with around 197mm offcut from each.
4.8m stock halves the number of lengths you need to buy for the joists. Combined with the 3.6m rim boards and 350mm blocking, the optimiser finds the overall best combination: which lengths to buy for which cuts, worked out automatically.
Blocking: Worth Including in the Cut List
Blocking is often treated as an afterthought, cut from offcuts on the day. But including it in the cut list upfront means the optimiser can plan for those 350mm lengths when selecting stock for the joists, potentially fitting them from the same lengths more efficiently.
- At 400mm joist centres with 4×2 (50mm thick) joists: blocking is 350mm
- At 400mm joist centres with 3×2 (50mm thick) joists: blocking is also 350mm
- At 600mm joist centres with 4×2 (50mm thick) joists: blocking is 550mm
A 4.8m length used for two 2300mm joists has around 194mm remaining after kerf, not enough for a 350mm block. But a 2.4m length used for one 2300mm joist leaves 97mm after kerf, also too short. The optimiser spots these interactions and adjusts the stock selection accordingly.
Kerf on a Deck Frame
The 3.6m × 2.4m example above involves at least 20 cuts across all the members. At 3mm kerf per cut, that is 60mm of material lost across the job, roughly equivalent to one blocking piece's worth of timber becoming sawdust.
Set your kerf before running the optimiser. 3mm suits most circular saws and mitre saws. If you are using a handsaw or a thinner-bladed chop saw, 2mm may be more accurate for your setup.
Free Plan or Pro for a Deck or Floor Frame?
The 3.6m × 2.4m example uses 4 cut rows and one timber size, well within the free plan. A larger deck with multiple timber sizes (for example 4×2 joists and 6×2 bearers), or additional blocking rows, may need more rows or sizes.
- Free plan: up to 8 cut rows, up to 2 timber sizes: covers most straightforward deck frames.
- 3-day Pro pass (£2): unlimited rows and timber sizes, PDF export: good for a weekend build.
- Monthly Pro (£5/month): useful if you are planning decks, floors or other framing jobs regularly.
Frequently Asked Questions
- What timber size is used for deck joists? Most domestic decks use 4×2 (100mm × 50mm) C16 regularised timber. Longer spans or heavier loads may need 6×2 (150mm × 50mm). Check span tables for your specific setup.
- How do I calculate the number of joists I need? Divide the deck length by the joist spacing and add one. For 3.6m at 400mm centres: 3600 ÷ 400 = 9 bays + 1 = 10 joists total, including rim boards.
- What length are interior joists? The deck width minus the thickness of both rim boards. For a 2.4m deck with 4×2 (50mm thick) rim boards: 2400 − 50 − 50 = 2300mm.
- What is blocking in a deck frame? Short members between joists at mid-span to prevent twisting. Length = joist spacing minus joist thickness. For 400mm centres with 4×2 joists: 400 − 50 = 350mm.
- What stock length is most efficient for 2300mm joists? 4.8m lengths give two joists each, halving the number of lengths you need. The optimiser calculates this automatically across all your cuts.