Decking Screw Calculator
Running out of decking screws mid-build means a frustrating trip back to the shop, while buying far too many ties up money unnecessarily. Enter your decking area and joist spacing below to get an accurate screw count.
Work out how many decking screws you need
Enter your decking area and joist spacing to get a screw count.
Including 10% wastage, you'll need approximately:
264 screws
That's 3 boxes of 100.
Recommended products
A few well-reviewed fixings and fasteners to consider.
- Nylon
£5.99
Pros
- Bulk 500+500 pack keeps a plug-and-screw combo on hand for every job
- Nylon plugs suit brick, block and masonry walls
- Trusted Rawlplug brand widely stocked by UK trade merchants
Cons
- No storage box included for the loose parts
- Steel
£17.70
Pros
- Assorted case covers the common sizes needed for flat-pack, joinery and general repairs
- Zinc/gold passivated steel resists corrosion in indoor use
- Compartmentalised case keeps sizes sorted and easy to find
Cons
- Not stainless, so unsuitable for damp or outdoor exposure
- Stainless Steel
£9.20
Pros
- A2 stainless steel resists rust for bathroom, kitchen and outdoor fixings
- Pozi-compatible countersunk head sits flush for a neat finish
- Box of 200 covers most small to medium DIY projects
Cons
- Single size per box means buying separately for mixed jobs
Frequently asked questions
Why does joist spacing affect how many screws I need?
Closer joist spacing means more fixing points running under the same area of decking, so a 300mm joist layout needs noticeably more screws per square metre than a 600mm layout, even though the board coverage is identical.
How many screws does each decking board fixing point need?
Most decking boards are fixed with two screws at each joist crossing, one near each edge of the board, which is the standard assumption built into typical screws-per-square-metre coverage rates.
Why add 10% extra screws on top of the calculated total?
A wastage allowance covers screws that snap, strip or get dropped and lost in the grass during the build, which is common enough on a decking project that it's worth buying slightly more than the bare calculated minimum.
Should I use different screws for hardwood versus softwood decking?
Yes — hardwood decking is much denser and usually needs harder, often stainless steel decking screws with a self-drilling or pilot-hole approach, whereas standard decking screws are generally fine for softwood boards.
How this calculator works
This calculator estimates the number of decking screws required by multiplying the perimeter of your deck (calculated based on its length and width) by the joist spacing. Joist spacing is a critical factor – standard spacings in the UK range from 600mm to 900mm, depending on the size and load-bearing requirements of your deck. The calculator then divides this total length by the diameter of the screw you’ve selected, accounting for the screw's head size and penetration depth into the joist.
The calculator assumes a straightforward rectangular deck design. For more complex shapes (e.g., curved decking or angled supports), you’ll need to break down the area into simpler sections to accurately estimate the total length of joists requiring screws. It also uses standard screw lengths – typically 60mm, 80mm, or 100mm – based on common decking applications.
Screw Size & Material Guide (UK)
- **BS EN ISO 284:2003 Standards:** UK decking projects overwhelmingly use screws conforming to this British Standard, ensuring compatibility and structural integrity.
- **Coated Screws:** Consider stainless steel or ceramic-coated screws for maximum corrosion resistance – vital in the UK’s often damp weather. Prices range from £8 - £15 per box (approximately 200 screws).
- **Screw Lengths:** 60mm is suitable for thinner decking boards, 80mm is a good all-rounder (£7 - £12 per box), and 100mm provides the greatest holding power for heavier timber or larger joists (around £9 - £14 per box).
- **Head Type:** Flat head screws provide a cleaner finish and better contact with the decking, while pan head screws offer more grip. Pan heads are generally recommended for increased security.
- **Timber Type:** Softwoods like pine require slightly coarser threads than hardwoods like treated timber to ensure adequate bite and prevent stripping. Consider this when selecting screw type.
Worked example: Standard 4m x 3m Deck
Let's calculate the number of 80mm ceramic-coated decking screws needed for a deck measuring 4 metres by 3 metres with joist spacing at 600mm. The perimeter is (4m + 3m) * 2 = 14 metres. Assuming a joist spacing of 600mm, we have 14 / 0.6 = 23.33 joists. Therefore, you'll need approximately 23 or 24 screws per joist to account for overlap and potential variations.
Using an 80mm screw (diameter 8mm), the total number of screws required is roughly 23.33 x 2 = 46.66. Rounding up, you’d need approximately 47 screws. The cost of a box of 200 ceramic-coated 80mm decking screws from Screwfix currently averages around £11 – a worthwhile investment for a robust and long-lasting deck foundation.
More questions answered
What if my deck isn't perfectly rectangular?
Break your deck down into simpler shapes like rectangles or triangles. Calculate the perimeter of each individual section and add them together to get the total joist length. This will provide a more accurate screw estimate.
How does joist spacing affect the calculation?
Tighter joist spacing (e.g., 450mm) requires significantly more screws due to increased joist length. Conversely, wider spacing reduces the screw count but may compromise structural integrity if not adequately supported; consult a professional for larger spans.
What happens if I use different sized screws?
The calculator’s output will change dramatically based on screw diameter and length. Larger screws require fewer but provide less holding power, while smaller screws need more to achieve the same level of security – always prioritize structural integrity.
Does this calculator account for decking board overlap?
No, this calculator focuses solely on joist screw requirements. Decking board overlaps and fixings between boards require separate calculation or use of a dedicated overlapping deck fixing calculator.
What is the minimum number of screws I should use per joist?
As a general guideline, aim for at least 2-3 screws per joist for standard decking applications. For heavier timbers or increased load requirements, increase this to 4 or more – consult a qualified engineer.
Can I use this calculator with composite decking?
Yes, but composite decking often requires specialist screws designed for composite materials. Ensure you select the correct screw type and length for optimal performance; check manufacturer recommendations.
What if my joists aren’t perfectly straight?
This calculator assumes straight joist runs. Any deviation will increase the total joist length, leading to a higher screw count. Accurate measurement is critical.
Does this calculator include any safety factors?
No, this calculator provides an estimate based on standard practices. Always incorporate a safety factor and consider consulting with a qualified engineer or structural surveyor for complex projects or high-load applications.
What if I'm unsure about my joist spacing?
Consulting with a professional is highly recommended, especially for larger decks or challenging ground conditions. Incorrect joist spacing can significantly impact the stability and longevity of your deck.
Where can I buy the screws calculated by this tool?
The calculator links to Amazon UK product listings where you can purchase the most commonly used decking screws - typically 80mm ceramic-coated screws, but always check compatibility with your decking material and project requirements.