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Free window & door fitter calculator
Window fitting calculator: frame size to order, cills, fixings, foam and trickle vents
Measure the openings, count the windows and doors going in and get the fitting materials: the frame size to order, cills and end caps, internal and external trims, frame fixings and packers, foam, silicone, trickle vents and door thresholds. It uses the same approach as TradeCalc in the Hard Hat Way app, with the fixing rule from the LABC Warranty and Liniar guides and vents set per room under Part F 2021.
What you need
Frames
- Frame size to order
- 1190 × 1160 mm1200 × 1200 mm opening less 10 mm tolerance and a 30 mm cill under the frame; add-ons and bay poles extraLiniar: 5 mm expansion gap all round; LABC: 5–10 mm, 15 mm on patio doors
- Frame perimeter
- 33.9 mfour sides of each window, three of each door; drives trims, foam and silicone
- uPVC windows
- 6 units1190 × 1160 mm, 8.3 m² glazed
- Composite door sets
- 1 set
Cills
- uPVC cill, 150 mm × 3 m
- 3 × 3 m lengths6 pieces at 1390 mm (frame plus 200 mm for horns), 2 per lengthLiniar installation guide: cut 200 mm over and form horns; LABC: 25 mm projection with a drip
- Cill end caps
- 6 pairsif square-cut instead of horned; horn gaps sealed, max 10 mm
Fixing
- Frame fixings, 7.5 × 132 mm
- 2 × boxes of 5062 fixings: 9 a window (3 a jamb, 3 in the cill, 0 in the head), 8 a doorLABC Warranty 8.1.1; Liniar: 150–250 mm from corners, 600 mm centres, two per side minimum
- Frame packers
- 1 pack of 100a packer behind every fixing, bridging the full frame widthLiniar installation guide
Sealing
- Gun foam, 750 ml
- 2 cansone can per 18 m at a 5–10 mm gapSoudal window and door foam: 21 m linear yield (FEICA TM1002)
- Low-modulus silicone
- 7 tubesone tube per 6 m inside and out, with 10% waste
- Internal trims or architrave
- 15 × 2.5 m lengthsthe full perimeter with 10% waste
- External trims
- 9 × 2.5 m lengths60% of the perimeter with 10% waste
Ventilation
- Trickle vents
- 11 vents2 per habitable room or kitchen (8,000 mm²) × 5, 1 per bathroom (4,000 mm²) × 1; set per room, not per window, and never fewer than the old windows hadApproved Document F 2021 (from 15 June 2022)
Doors
- Door cills or thresholds
- 1 eachLABC: threshold at least 150 mm above ground unless level access
- Multipoint lock and cylinder sets
- 1 set
Rules
- Part L
- 1.4 W/m²K or WER B replacement windows must reach 1.4 W/m²K or Window Energy Rating band B; doors 1.4 or DSER B (C if more than 60% glazed)Approved Document L 2021 (BWF summary)
- Notify
- FENSA, CERTASS or building control replacement windows and external doors are notifiable in England and Wales: a competent-person installer self-certifies and registers it, otherwise apply to building control. Check the DPC and cavity closer when the old frame comes outgov.uk building regulations approval; FENSA
Round up to whole packs and bags. Figures include the wastage you chose and match TradeCalc in the app.
For window & door fitters: turn this into a quote
In the app, TradeCalc hands these quantities straight to ProQuote with your regional material prices, and the customer gets a branded PDF. Free to download.
Or claim your free profile link nowHow the maths works
- Frame size to order is the brick-to-brick opening less the fitting tolerance: Liniar allows a 5 mm expansion gap all round, so 10 mm off the width and the height (LABC: 5–10 mm for uPVC, up to 15 mm on patio doors), and a further 30 mm off the height when a cill goes under the frame. Measure in three places each way, use the smallest and check the diagonals.
- Every window is taken at the average size you enter, external doors at 900 × 2100 mm and patio, French or bifold sets at 2400 × 2100 mm. Frame perimeter is four sides of each window and three of each door and drives the trims, foam and silicone.
- Cills are cut 200 mm longer than the frame so a horn can be formed at each end (or square-cut with end caps), and the calculator counts how many pieces come out of a 3 m or 6 m length rather than dividing metres. Cills come 85, 150, 180 and 225 mm deep and must project at least 25 mm past the wall with a drip.
- Fixings follow LABC Warranty 8.1.1 and the Liniar guide: 150–250 mm from each corner, none within 150 mm of a mullion or transom, at most 600 mm centres, so at least two in each jamb and the cill. With a foamed head there are no head fixings up to 1,200 mm wide, one to 2,400 mm and two to 3,600 mm. Doors get jambs only; patio and bifold sets follow the maker. A packer sits behind every fixing, bridging the frame, and the fixing goes at least 40 mm into the masonry.
- Gun foam is one 750 ml can per 18 m of frame at a 5–10 mm gap, from the Soudal window and door foam yield of 21 m. Low-modulus silicone is one tube per 6 m inside and out plus 10%. Internal trim covers the full perimeter and external trim 60% of it, in 2.5 m lengths with 10% waste.
- Trickle vents are set per room under Part F 2021, not per window: 8,000 mm² equivalent area for each habitable room and kitchen and 4,000 mm² for each bathroom, and never fewer than the old windows had. A standard vent is about 4,000 mm², so most rooms need two or one large one.
- Replacement windows must reach 1.4 W/m²K or WER band B (Part L 2021) and are notifiable: a FENSA or CERTASS installer self-certifies, otherwise building control.
Worked example
Six 1200 × 1200 mm openings with 10 mm tolerance and a cill under the frame take 1190 × 1160 mm frames. With one composite door that is 33.9 m of frame perimeter: 3 lengths of 150 mm cill (six 1390 mm pieces with horns, two per 3 m), 2 boxes of frame fixings (62 in all, 9 a window), 2 cans of foam, 7 tubes of silicone, 15 lengths of internal trim and 9 external, and 11 trickle vents for five habitable rooms and a bathroom.
Questions people ask
How do I measure for replacement windows?
Measure the brick-to-brick opening width and height in three places each, use the smallest, and check the diagonals are equal. Measure inside and out: the outside opening is often smaller because of the plaster reveal, and note the reveal depth for add-ons. The frame is ordered 10 mm smaller than the opening (5 mm all round) and 30 mm shorter again if a cill goes under it, so it goes in square and can be packed. If you are ordering yourself, have the supplier survey it.
Do new windows need trickle vents?
Yes, under Part F 2021 (from 15 June 2022) in England: replacement windows need background ventilation of 8,000 mm² equivalent area in each habitable room and kitchen and 4,000 mm² in each bathroom, and never less than the old windows gave. That is per room, not per window, so a room with one window usually needs two standard 4,000 mm² vents or one large one. The vents are routed into the frame at the factory, so decide before you order.
How many fixings does a window need?
Work along each side: one 150–250 mm from each corner and then no more than 600 mm apart, so a 1,200 mm jamb takes three and a 900 mm one takes two. Never within 150 mm of a mullion or transom. If the head is foamed rather than fixed there are no head fixings up to 1,200 mm wide. Put a packer behind every fixing so the frame is not pulled out of shape, and use a fixing long enough to go 40 mm into the masonry.
What U-value do replacement windows need?
Part L 2021 sets 1.4 W/m²K or a Window Energy Rating of band B for replacement windows, 1.4 or DSER B for doors (DSER C if more than 60% glazed). A plain air-filled double unit will not get there; you need a low-E coating and argon fill, which any FENSA or CERTASS installer will supply as standard.
Do I need building control for new windows?
Replacement windows and external doors are notifiable in England and Wales. An installer registered with FENSA, CERTASS or another competent person scheme self-certifies the work, registers it with the council and gives you a certificate with an insurance-backed guarantee; otherwise you have to apply to building control and pay for an inspection. You will be asked for the certificate when you sell.
uPVC, aluminium or timber?
uPVC is the cheapest and needs least looking after. Aluminium gives slimmer frames and bigger panes and suits modern houses and bifolds. Timber suits period houses and conservation areas but needs painting or staining. In a conservation area or on a listed building check with the council first.
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