Sash Weight Calculator: How to Work Out the Weight Your Sash Windows Need

If you’re replacing or rebalancing the weights in a box sash window, the first question is always the same: how heavy should each weight be? Get it right and the sash glides up and down and stays exactly where you leave it. Get it wrong and the window either drops shut on its own or won’t stay down. The good news is there’s a simple rule and a bit of arithmetic behind it, and you can work out the answer yourself with nothing more than a set of scales. This is the manual method a sash window weights calculator follows under the bonnet.

The balancing principle

A box sash window works on a counterweight system. Inside the hollow box frame on each side of the window hangs a cast weight (traditionally lead, sometimes cast iron or steel) on a sash cord that runs up over a pulley at the top. The weight pulls down on one end of the cord while the sash pulls down on the other, and when the two are matched the sash effectively becomes weightless. That’s why a well-balanced sash stays put at any height.

So the rule is this: the total counterweight on each side should roughly equal the weight of the sash it’s lifting. Each opening sash (the glazed timber frame, including the glass) is carried by two cords, one on each side, with a weight on each cord. So the maths is straightforward:

Total weight needed for a sash = the weight of that sash.
Each individual weight = about half the sash’s weight (because there are two of them, one per side).

That “about” matters. You don’t want the weights to be heavier than the sash, or the window will creep upwards on its own. A touch under is usually safer than over, because cord and pulley friction holds the sash in place across small differences. Aim to match, and trim from there.

One more thing before the steps: the top and bottom sashes of a sliding sash window are usually different weights, and they often use different-sized counterweights. Always work out each sash separately.

How to work out the weight you need

Method A — weigh the actual sash (the accurate way)

Nothing beats weighing the real thing, because it accounts for the exact glass, timber and any layers of old paint.

Remove the sash from the frame. With the beads off and the cords detached, lift the sash out. Take care: a large double-glazed sash is heavy and awkward, and old timber can be fragile. If you’re not confident, get a second pair of hands or call a joiner.
Weigh it. Stand on bathroom scales holding the sash, then weigh yourself alone and subtract the difference. For smaller sashes, luggage scales hooked under the bottom rail work well and are more precise.
Halve it. Divide the sash weight by two to get the weight each side needs.

That figure is your target per-weight mass. If your old weights are still in the box, weigh them too and compare — it’s a quick sanity check on whether the window was correctly balanced before.

Method B — estimate from glass and size (when you can’t weigh it)

If the sash is still in place or you’re buying ahead, you can estimate. It won’t be exact, but it gets you into the right bracket.

The bulk of a sash’s weight is the glass, and glass weight depends heavily on type:

Single glazing (3–4 mm float glass): roughly 7.5–10 kg per square metre of glass.
Double glazing (a slim or standard unit): roughly 20–25 kg per square metre — often more than double the single-glazed figure.

Work out the glass area (height × width in metres), multiply by the figure above, then add a bit for the timber frame — a rough 2–4 kg for a typical Victorian sash, more for a big or chunky one. That total is the sash weight; halve it for each side.

Treat Method B as a starting point only. The single-versus-double-glazing difference is the thing that catches people out, so if the window has ever been re-glazed, the original weights are almost certainly wrong.

A worked example

Say you take a lower sash off and weigh it on the bathroom scales at 12 kg.

Total counterweight needed = the sash weight = 12 kg.
That’s split across two cords, so each weight = 12 ÷ 2 = 6 kg.

So you’d be looking for a pair of 6 kg weights for that sash. In old money that’s about 13¼ lb each, which is well within the normal working range for sash weights.

Now an estimate using Method B, for a single-glazed sash measuring 0.5 m wide × 0.9 m high:

Glass area = 0.5 × 0.9 = 0.45 m².
Glass weight at ~9 kg/m² = 0.45 × 9 = ~4 kg.
Add ~3 kg for the timber frame = ~7 kg total sash weight.
Each weight = 7 ÷ 2 = ~3.5 kg.

Same window with double glazing instead would push the glass to around 0.45 × 22 = ~10 kg, giving a ~13 kg sash and ~6.5 kg per side — nearly double. That gap is exactly why you can’t reuse Victorian weights after a double-glazing upgrade without checking.

Adjusting for double glazing and thick old glass

Heavier glass needs heavier weights, full stop. If you’ve replaced single panes with sealed double-glazed units, or your original glass is the thick, slightly wavy old kind, your sash is heavier than a standard build and your counterweights must go up to match. Skip this and the new, heavier sash will slam shut every time you let go.

This is where the material of the weight starts to matter, because the box pocket the weight slides in is narrow and only so long. You need to fit more mass into the same small space:

Lead is the densest common option (around 11.3 g/cm³), so a lead weight is the shortest and slimmest for a given mass — ideal when you need extra kilograms in a tight box. It’s also soft, so you can shave a little off to fine-tune the balance.
Cast iron (around 7.2 g/cm³) and steel (around 7.85 g/cm³) are less dense, so the same mass takes up noticeably more length and may not fit a deep-enough drop.

If you’re going heavier for double glazing and space is tight, lead usually makes the job possible where iron won’t fit. We go through the trade-offs in full in our guide to lead vs cast iron vs steel sash weights.

From “how many kg” to “what it costs”

Once you know the weight you need — say a pair of 6 kg lead weights — you can buy on the one thing that actually lets you compare suppliers fairly: price per kilogram (£/kg). Suppliers quote in all sorts of ways (per piece, per pound, per kilo, some ex-VAT for trade, some inc-VAT for consumers), which makes raw prices almost impossible to line up. Normalised to £/kg, the picture is clear — and there’s a genuine two-to-three-and-a-half-times spread between the cheapest and dearest UK suppliers for the same lead. Buying badly can cost you double for identical metal.

That’s the whole reason this site exists. Once you’ve got your figure in kilograms, you can compare live £/kg prices from UK suppliers and pick the cheapest within the right size and section for your window. To translate kilograms into the standard weight sizes suppliers actually stock (25, 32, 38, 44 mm and so on), our sash weight size chart does the lookup for you.

Common mistakes to avoid

Under-weighting the sash. Weights that are too light won’t hold the sash up — it’ll slide down on its own. Match the sash, don’t undershoot it badly.
Ignoring a glazing change. The single biggest error. If the glass was swapped from single to double at any point, the old weights are too light. Always recalculate from the current glass.
A mismatched pair. Both weights on one sash should be equal, or the sash will rack and bind in the runners. Buy matched pairs and check them on the scales.
Forgetting cord and pulley friction. A small amount of friction is normal and actually helps the sash stay put. Don’t add extra weight to “overcome stiffness” — sort the sticking sash, pulley or paint build-up first, or you’ll end up with a window that creeps up.
Reusing whatever was in the box. Old weights may have been wrong for decades, or belong to a different sash entirely. Weigh, don’t assume.

FAQ

How do I calculate sash window weights? Weigh the sash (glass and frame together), then divide by two — there are two weights per sash, so each one carries half. If you can’t weigh it, estimate from the glass area and type, add a few kilograms for the timber, then halve the total.

Should each weight be exactly half the sash weight? Close to it. The pair together should match the sash; each one is roughly half. Err very slightly under rather than over, because cord and pulley friction takes up the small difference and stops the sash creeping upward.

Do I need heavier weights for double glazing? Almost always, yes. Double-glazed units can weigh more than twice as much per square metre as single glazing, so a re-glazed sash needs heavier counterweights. Lead is the usual choice here because its density lets you fit the extra mass into the narrow box.

What if my old weights are missing or unmarked? Work from the sash, not the weights. Weigh the sash and halve it — that gives you the target regardless of what was there before. Then compare £/kg prices to buy a matched pair.

Working out your figure

The method is simple: match the total counterweight to the sash, split it across two cords, and lean slightly under rather than over. Weigh where you can, estimate carefully where you can’t, and always recheck after any change to the glass. Once you’ve got your figure in kilograms, compare live £/kg prices from UK suppliers to buy the right weight for the least money — we’ve done the price-checking so you don’t have to.