Lead vs Cast Iron vs Steel Sash Weights: Which Should You Buy?
If you’ve opened up a box sash window to re-hang or replace a weight, you’ll quickly meet a question that the old joiners settled long ago but that still trips people up today: what should the weight actually be made of? The counterweights inside a box sash come in three main materials — lead, cast iron and steel — and you’ll also come across reclaimed or antique weights pulled from older houses. They all do the same job, but they don’t fit the same, cost the same, or suit the same projects. This guide walks through each one honestly so you can pick the right material for your window, then compare what it should cost.
The three materials you’ll meet
Almost every sash weight you’ll handle is one of these:
- Lead — the traditional choice, still the default for restoration work.
- Cast iron — common in Victorian and Edwardian houses, historically the cheaper option.
- Steel — a modern substitute, used mainly where lead isn’t available.
On top of those, there’s a fourth category worth knowing about: reclaimed or antique weights salvaged from other windows. Those can be any of the three materials, and they come with their own trade-offs. We’ll cover all four below.
Quick verdict: lead vs cast iron vs steel
Here’s the short version before we get into the detail. The density figures matter more than they look — they decide how much weight fits inside a narrow box pocket.
| Material | Density (g/cm³) | Pros | Cons | Best for |
|---|---|---|---|---|
| Lead | ≈ 11.3 | Densest, so most mass in least space; soft and easy to cut/trim; corrosion-resistant | Needs careful handling; usually the dearest per kg | Heritage restoration, tight box pockets, fine balancing |
| Cast iron | ≈ 7.2 | Period-correct for Victorian houses; often cheap second-hand | About 36% less dense than lead, so bulkier; can rust | Matching existing cast-iron weights |
| Steel | ≈ 7.85 | Modern, widely available, functional | Least dense of the three, so needs the most length | Budget jobs where lead isn’t to hand |
Lead — the traditional choice
For most sash windows, lead is still the best material, and there’s good reason it became the standard.
The big advantage is density. At around 11.3 g/cm³, lead is far heavier for its size than cast iron or steel. A box sash has only a narrow hollow pocket on each side of the frame for the weight to travel up and down, and a heavy sash (think old thick glass, or a sash you’ve upgraded to double glazing) needs a lot of counterweight. Lead lets you fit the most mass into that limited space, which is exactly why it’s the go-to for heavier sashes and slim original boxes.
Lead is also soft and easy to work. You can saw, file or shave a lead weight down to fine-tune the balance, so each side matches the sash precisely. That matters because the rule of thumb is that the weights on each side should together roughly equal the weight of that sash, split between the two cords — so each weight carries about half. Being able to trim to the gram makes lead forgiving to balance.
Finally, lead is corrosion-resistant. It won’t rust away inside a damp box pocket the way iron or steel can, which is part of why so many original lead weights are still serviceable a century or more on.
A word on handling and safety. Lead is a toxic metal, but solid sash weights are low-risk if you treat them sensibly: wash your hands after handling them, don’t eat or smoke while you work, and never sand, grind or burn bare lead — that’s what creates harmful dust and fumes. Keep offcuts away from children and pets. None of this should put you off; it’s the same common sense any tradesperson uses.
Cast iron — common in older houses
If your house is Victorian or Edwardian, there’s a fair chance the original weights are cast iron. It was historically cheaper than lead, so it turned up in a lot of period building.
The catch is density. Cast iron sits at around 7.2 g/cm³ — roughly 36% less dense than lead. That means a cast-iron weight has to be noticeably bulkier to match the mass of a lead one. In some windows that’s fine; in others, a cast-iron weight that’s heavy enough simply won’t fit, or it can be too fat to run cleanly up and down the box pocket without catching. Cast iron also rusts, so weights that have sat in a damp frame can be scaled and pitted, and badly corroded ones lose mass over time.
Where cast iron earns its place is matching. If a window already has three good cast-iron weights and you only need a fourth, buying or reclaiming a matching cast-iron weight keeps everything consistent in size and behaviour. For a sympathetic period repair, like-for-like is often the right call.
Steel — the modern substitute
Steel weights are the modern stand-in, used mainly where lead isn’t readily available. They’re functional and widely sold, and for a straightforward repair they’ll do the job.
The drawback is that steel is the least dense of the three, at around 7.85 g/cm³. To hit the same mass as a lead weight, a steel one needs to be the longest of all — which can be a problem in a shallow box where there’s limited travel for the weight to drop. Steel can also rust if it’s not coated and the frame gets damp. It’s perfectly serviceable, but it’s rarely the first choice for heritage work, where lead’s fit and authenticity win out.
Reclaimed and antique sash weights
There’s a thriving second-hand market in old sash weights, and it’s genuinely useful — especially when matching originals matters more than buying new.
You’ll find reclaimed and antique sash weights on eBay UK and at architectural salvage yards. The appeal is real: they’re authentic, period-correct, and sometimes very cheap, particularly if a yard is clearing a job lot. For a restoration where you want the same casting marks and the same feel as the rest of the window, salvage can be ideal.
The honest downsides: reclaimed weights have inconsistent mass. They were cast in lots of different sizes, and a rusted iron weight may weigh less than its size suggests. There’s no guarantee and no return if one turns out wrong. And you’ll usually have to weigh and match them yourself — meaning your time, a set of scales, and possibly a wasted trip. Reclaimed weights suit patient, matching-led jobs more than a quick, certain replacement.
What density means in practice
Density is the thread running through all of this, so it’s worth seeing it concretely.
A box sash pocket is narrow and only so deep. Whatever weight you choose has to fit inside it and travel freely. The denser the metal, the more mass you pack into that fixed space — which is exactly why lead suits tight, original boxes.
Here’s a rough illustration. Imagine you need a weight of a given mass, and a lead weight of the right section is, say, 30 cm long. To hit the same mass:
- A cast-iron weight of the same section would need to be roughly 47 cm long (cast iron is about 7.2/11.3 as dense, so it needs about 1.57× the volume).
- A steel weight would need to be around 43 cm long (steel at 7.85 needs about 1.44× the volume).
So a steel or cast-iron replacement can end up nearly half as long again as the lead weight it replaces. In a deep box that’s a non-issue; in a shallow one, it can be the deciding factor.
Which should you choose?
A quick decision guide by scenario:
- Heritage restoration: go with lead. It’s the historical standard, fits slim original boxes, and trims for precise balance.
- Tight or shallow box pocket: lead, every time — its density is the whole point.
- Budget repair, lead not to hand: steel or cast iron will work; just check there’s enough room for the extra length.
- Matching existing weights: match the material that’s already there. If the window has cast iron, add cast iron (new or reclaimed) for consistency.
When in doubt, weigh the sash and the existing weights before you buy, and if the timber’s fragile or the window’s large and heavy, call a joiner rather than guessing.
The cost angle — compare on £/kg
Here’s the practical bit that ties it together. Sash weights are mostly priced by weight, so the only fair way to compare suppliers is £ per kilogram (£/kg) — not per piece, which hides how much metal you’re actually getting.
It’s worth shopping around, because there’s a genuine 2 to 3.5 times spread between the cheapest and dearest UK suppliers for the same lead. Prices also move with the lead market, so they’re not fixed. Rather than us quoting figures that go stale, you can compare live £/kg prices from UK suppliers on our homepage, where every supplier is normalised to the same £/kg basis and ranked cheapest first.
For the wider picture on what weights cost and why, see our sash weights cost and UK price guide. And if you’re not sure how much weight you actually need before you buy, our sash weight calculator guide walks you through balancing the sash.
FAQ
Are old sash weights lead or cast iron? Both are common. Many Georgian and earlier sashes used lead, while a lot of Victorian and Edwardian houses used cast iron because it was cheaper at the time. The easiest tell is weight for size: for the same dimensions, lead is much heavier. Lead also looks dull grey and is soft enough to mark with a knife; cast iron is harder and often rusty.
Can I mix lead and cast iron weights? You can, as long as the maths and the fit both work. What matters is the total mass on each side, not the metal — so a lead weight on one side and a cast-iron one on the other is fine if each side is correctly balanced to the sash. Just check the bulkier cast-iron or steel weight physically fits and runs freely in its box pocket.
Are lead sash weights safe? Yes, for normal use. Solid lead weights sitting inside a sealed box frame pose very little risk. The sensible precautions are simple: wash your hands after handling them, don’t sand, grind or burn bare lead, and keep offcuts away from children and pets. The danger comes from lead dust and fumes, not from a solid weight hanging in a window.
Which is best for a heavy double-glazed sash? Lead, almost always. A double-glazed sash is heavier than the single glazing it replaced, so it needs more counterweight — and lead packs the most mass into the same box pocket. Steel or cast iron may simply be too bulky or too long to fit the extra weight in.
Compare prices before you buy
Once you’ve decided on a material — and for most jobs that’s lead — the next step is making sure you don’t overpay. Because suppliers quote in different ways, the same lead can cost two to three times more from one merchant than another. Compare live £/kg prices from UK suppliers on our homepage to find the best value, normalised so you’re comparing like for like.