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Glass weight calculator: sealed unit size, weight, safety glass and lifting
Work out the size of glass or sealed unit to order for an opening, what it will weigh and what you need to glaze it. It uses the same approach as TradeCalc in the Hard Hat Way app: 5 mm edge clearance off the rebate tight size, weight from the pane thicknesses at 2.5 kg per m² per millimetre, a check against the Part K critical locations for safety glass, and the HSE lifting bands.
What you need
Glass
- Unit weight
- 23.6 kg20.0 kg/m² over 1.18 m²2.5 kg/m² per mm (2,500 kg/m³); laminated 2.48; wired 2.57
- Size to order
- 1190 × 990 mmrebate tight size less 5 mm edge clearance each sideNHBC 6.7.5: 3–5 mm clearance, 12 mm edge cover, 15 mm rebate
- Make-up 4-20-4
- 28 mm thick4 mm float / 4 mm float, 20 mm spacer
- Pane 1: 4 mm float
- 11.8 kg10.0 kg/m²
- Pane 2: 4 mm float
- 11.8 kg10.0 kg/m²
- Units to order
- 1 unitdouble glazed, low-E with argon, float; give the glazier the make-up and which panes are toughened
Rules
- Lift
- 1 person under the HSE 25 kg guideline for one person at waist height (16 kg for women, 10 kg above the shoulder)HSE INDG143 manual handling guideline weights
- Safety glass
- Not a critical location Part K applies below 800 mm in a wall, below 1,500 mm in a door and within 300 mm of a door edge; bathrooms are not a Part K location, and shower screens are covered by BS 6262-4Approved Document K, Section 5
- Part L
- 1.4 W/m²K or WER B a replacement window must reach 1.4 W/m²K or WER band B as a whole window; a low-E argon unit is about 1.1–1.2 centre paneApproved Document L 2021 (BWF summary)
Fitting
- Glazing bead
- 4.6 m2 × 3 m lengths, with 5% waste
- Wedge gasket or seal
- 9.2 mboth sides, with 5% waste
- Silicone
- 1 tubeone per 6 m of perimeter
- Glazing packers and setting blocks
- 4 packersfour a unit, toe and heel on an opener
Round up to whole packs and bags. Figures include the wastage you chose and match TradeCalc in the app.
For glaziers: 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
- Size to order is the rebate tight size less the edge clearance on each side: 5 mm off each side takes 10 mm off the width and the height. NHBC 6.7.5 gives 3–5 mm edge clearance with at least 12 mm edge cover and a 15 mm rebate. Measure the rebate tight in three places and use the smallest.
- Weight is built from the panes: 2.5 kg per m² for every millimetre of float, toughened or patterned glass (2,500 kg/m³), 2.48 for laminated plus 0.2 kg/m² for the interlayer, 2.57 for Georgian wired. A 4-20-4 unit is 20 kg/m², a 6-16-6 is 30, a 4-16-4-16-4 triple is 30. The unit thickness is the panes plus the spacers.
- Safety glass is checked against Approved Document K: glass below 800 mm in a wall, below 1,500 mm in a door, or within 300 mm of a door edge is a critical location and needs Class C (3(B)3) to BS EN 12600, or Class B if the smaller dimension is over 900 mm. A pane under 250 mm wide and 0.5 m² may be 6 mm annealed. Bathrooms are not a Part K location; shower screens are covered by BS 6262-4.
- Lifting follows HSE INDG143: 25 kg for one person at waist height (16 kg for women, 10 kg above the shoulder), about 33 kg for two people (a team lift is not double), and suckers or a vacuum lifter above that. Anything over 1.5 m tall is flagged as awkward.
- Part L 2021: a replacement window must reach 1.4 W/m²K or WER band B as a whole window. A plain air-filled double unit is about 2.7 W/m²K centre pane; a low-E argon unit about 1.1–1.2; single glazing about 4.8.
- Glazing bead is the perimeter plus 5% waste in 3 m lengths. Wedge gasket or seal is both sides plus 5%. Silicone is one tube per 6 m. Packers are four a unit. Sprigs, for putty and timber single glazing, are one every 200 mm.
Worked example
A 1200 × 1000 mm rebate with 5 mm clearance each side takes a 1190 × 990 mm unit of 1.18 m². As a 4-20-4 low-E unit (28 mm) it weighs 23.6 kg, under the 25 kg one-person guideline, with its bottom edge 900 mm up in a wall so no safety glass is needed. Fit it with 4.6 m of bead (two 3 m lengths), 9.2 m of gasket, one tube of silicone and four packers.
Questions people ask
How much does glass weigh per square metre?
About 2.5 kg for every millimetre of thickness, so 4 mm float is 10 kg per m², 6 mm is 15 kg, 10 mm is 25 kg. A sealed unit is the sum of its panes: a 4-20-4 is 20 kg/m², a 6-16-6 toughened unit 30 kg/m², a 4-16-4-16-4 triple 30 kg/m². Laminated is fractionally lighter per millimetre (2.48) plus the interlayer; Georgian wired is heavier (2.57).
How do I measure for a replacement double glazed unit?
Take the beads off and measure the old unit if you can: width, height and overall thickness. If not, measure the rebate tight in three places each way and use the smallest; the unit size is the tight size less twice the edge clearance (5 mm each side). Note the make-up (4-20-4 is 28 mm), whether the spacer is warm-edge or aluminium, and whether the panes are toughened, low-E or obscure. Get the glazier to check anything in a door.
When does glass have to be toughened or laminated?
Approved Document K sets the critical locations: any glass below 800 mm from the floor in a wall or partition, below 1,500 mm in a door, and within 300 mm of a door edge below 1,500 mm. It has to be Class C (3(B)3) to BS EN 12600, or Class B (2(B)2) if the smaller dimension is over 900 mm; a small pane under 250 mm and 0.5 m² may be 6 mm annealed. Bathrooms are not a Part K location as such, though shower screens must meet BS 6262-4 and most glaziers toughen bathroom glass anyway.
How heavy is too heavy to lift?
The HSE guideline for one person is 25 kg at waist height (16 kg for women) and 10 kg above the shoulder. A two-person lift is capped at about 33 kg, not 50, because you cannot both get a good hold. Above that use suckers and a third person or a vacuum lifter, and plan the route before the glass arrives. A 1190 × 990 mm 4-20-4 unit is about 24 kg; a 6-16-6 toughened unit the same size is 35 kg.
Can I replace a blown double glazed unit myself?
On a uPVC window with internal beads it is a straightforward job if the unit is a size you can handle: measure, order, remove the beads, lift out the old unit, set the new one on packers, toe and heel it and refit the beads. Anything heavy, high up or in a door is a job for a glazier. Swapping a unit like for like is not notifiable; replacing whole windows is, and a replacement window has to meet Part L (1.4 W/m²K or WER B).
How much does a new double glazed unit cost?
It depends on the size, the make-up and whether it is toughened, laminated, low-E or obscure, and prices vary by area. Get a few quotes from local glaziers with the exact size and make-up.
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