Sash Window Weight Calculator (kg) and Replacement Lead Weights
Sash window weight calculator in kilograms
Work out what the counterweights in a double hung window should weigh, per sash and per weight, from measurements you can take with the window shut. Already have weights in the pockets? Enter what they weigh, in kg or pounds, and the calculator gives the difference to add.
Your window
W sash width · T top sash height · B bottom sash height. Measure the sash itself, not the frame opening.
Enter what one existing weight weighs and the calculator works out the difference to add. Old iron weights carry Roman numerals cast into them: the numeral is the nominal weight in pounds. The key beside the results decodes them.
How this is calculated
Glass: sash size minus the frame members, times the glass weight per square metre for your glazing. Float glass weighs 2.5kg per square metre for every millimetre of thickness.
Timber: stile and rail volumes at standard Australian sections, times timber density. Glazing bars add about 10 percent.
Balance: each sash hangs on two weights, so each weight carries half the sash. The top sash gets a small extra bias so it stays up, and the bottom sash the same bias in reverse so it stays shut.
It is still an estimate. If the sash is already out, weigh it on bathroom scales and use that number instead.
Top sash
Bottom sash
Order sizes
Our lead sash weights come as 0.25kg, 0.5kg and 1kg lots with a centre hole, stacked on the cord to make up each weight. The calculator rounds each weight to the nearest 0.25kg, so the order lands within 125g of the target and the cording takes care of the rest.
Key: Roman numeral stamps on old iron weights
The numeral cast into an old weight is its nominal weight in pounds. Old foundries often cast additive numerals, so IIII rather than IV and VIIII rather than IX.
The stamp is the nominal casting weight. Rust and casting variation shave a little off, so weigh one on kitchen scales when you can and enter the real number above.
The scales beat the calculator every time. If your sash is already out, weigh it on bathroom scales and order to that number instead.
Glass thickness
Float glass weighs 2.5kg per square metre for each millimetre. We take it from there.
Replacement sash weights, in lead
A round extruded lead weight with its centre hole, next to the old cast iron it replaces. Lead carries the same weight in roughly half the length.
Old double hung windows were balanced with cast iron bars hidden in pockets each side of the frame. When a window gets reglazed with heavier glass, or a weight has been lost or seized over the years, the balance goes and the window either crashes shut or will not stay up.
Lead is the material of choice for replacements because it is nearly twice as dense as cast iron. When heavier glass goes in, the pocket that held the old iron weight often cannot fit a longer one, and a lead weight of the same length carries far more. Lead does not rust, and the weights come drilled so the cord attaches the traditional way.
Our lead sash weights are round extruded bar with a centre hole, sold as 0.25kg, 0.5kg and 1kg lots that stack together on the cord. Any target figure comes together in 0.25kg steps, with no cutting, and the stacks run freely in standard Australian weight pockets. Because lead is heavy and soft, it ships by road freight only.
The three lots, unpeened: 0.25kg, 0.5kg and 1kg. They stack on the cord to reach any figure in 0.25kg steps.
How heavy should sash weights be?
Each sash hangs on two weights, and together they should weigh about the same as the sash. The trade habit is to balance the top sash slightly heavy, so it holds itself closed against the head, and the bottom sash slightly light, so it rests shut on the sill. The calculator above builds that bias in.
| Sash (820mm wide, 3mm glass) | Sash weight | Each counterweight |
|---|---|---|
| 700mm tall, softwood | about 8.5kg | about 4.5kg |
| 900mm tall, softwood | about 10.5kg | about 5.5kg |
| 700mm tall, reglazed with 6.38mm laminated | about 13kg | about 7kg |
The last row is the one that catches people out. Upgrading the glass roughly doubles the glass weight, and the original weights cannot balance it. Any reglaze to laminated or vacuum glazing should be priced with a set of heavier weights included.
Adding to weights you already have
If the pockets still hold their original iron, there is usually no need to replace it. Weigh one existing weight, or read its stamp, enter it in the calculator in kilograms or pounds, and the result switches from a full set to the difference: lead lots that thread onto the cord above the old iron and make up the shortfall. This is the usual path on a reglaze, where the old weights are sound and simply short by a few kilograms.
Reading the Roman numerals on old sash weights
Most Australian cast iron sash weights carry a Roman numeral cast into the side or the top. That numeral is the foundry's nominal weight in pounds: an VIII weight was cast as 8 pounds, which is about 3.6 kilograms. Two habits of old foundries trip people up. They often cast additive numerals, so 4 appears as IIII and 9 as VIIII, and the stamp is nominal rather than exact, since rust and casting variation shave a little off over a century. The key beside the calculator converts the common stamps to kilograms, and where accuracy matters the kitchen scales settle it.
Frequently asked questions
Can I reuse my cast iron weights? Yes, if the sash weight has not changed. Keep them if the window is only being resealed or re-corded. Replace them when the glass gets heavier or a weight is missing, or top them up with lead lots and order only the difference.
What do the Roman numerals on my old weights mean? Nominal pounds. X is 10 pounds, about 4.5kg. Foundries often used additive forms such as IIII for 4 and VIIII for 9. Weigh one when you can, since the stamp is nominal rather than exact.
Why lead instead of steel or iron? Density. Lead packs the same weight into roughly half the length, which matters because the weight has to travel the full pocket without hitting the pulley or the bottom.
How do I know the weight will fit my pocket? The pocket runs the height of the frame each side. The calculator shows the approximate stack length for each weight; as long as that is comfortably shorter than the sash height, it travels freely.
Why road freight only? Weight. A window's set is often 10 to 25kg of lead, which is outside express satchel limits and needs to be handled properly. It adds a few days, not weeks.
What if my sash is an odd size or the glass thickness is unusual? Use the Other option in the calculator and type the thickness in, or weigh the sash and order to the scales.