Can I measure ring size with string

Yes — but it is the least reliable of the common home methods, and it fails in a predictable direction. String almost always tells you your finger is smaller than it really is.

That matters because the two most likely outcomes are not equally bad. A ring slightly too large can be worn today and sized down later. A ring that will not pass your knuckle cannot be worn at all.

Why string underestimates

Two separate errors, both pushing the same way.

String stretches

Cotton thread, embroidery floss, dental floss and most craft string all have some give in them. Pull a length taut and it lengthens. When you then relax it and lay it flat against a ruler, it contracts again — so the number you read is shorter than the length that was actually wrapped around your finger.

Dental floss is especially prone to this, because it is engineered to be pulled hard without snapping. Waxed floss in particular can stretch appreciably under normal finger pressure.

String cuts into the skin

A narrow, hard cord concentrates all its pressure on a thin line. A 5 mm paper strip spreads the same tension across a wider contact patch. Wrap a thin cord snugly and it sinks into the flesh rather than resting on it, so you end up measuring a compressed finger rather than a relaxed one.

If string is genuinely all you have

It still beats guessing. Take these precautions and the result becomes usable as a starting point, though not as a final answer for anything expensive.

1. Choose the least stretchy thing available Unwaxed dental floss, fishing line or a strip cut from a plastic bag all stretch less than cotton thread. Better still, use paper — an envelope, a receipt, the margin of a page. The paper strip method
2. Wrap without pulling Bring the ends together until they just meet. Resist the instinct to snug it up; that instinct is exactly what causes the error.
3. Mark, do not tie A knot adds length and is impossible to measure consistently. Mark the crossing point with a fine pen while the string is still on the finger.
4. Check the knuckle before you take it off Slide the loop up over the knuckle at the marked length. If it will not pass, your knuckle is the limiting measurement and you need to remeasure there.
5. Measure the string flat, unstretched Lay it on the ruler without tension. Pulling it taut against the ruler reintroduces the stretch error you were trying to avoid.
6. Repeat at least three times and take the largest With most methods you average. With string you take the largest reading, because the errors are one-directional — the biggest number is the one least affected.
7. Treat the result as a floor, not an answer If your reading lands right on a size boundary, go up rather than down.

How string compares to the alternatives

Ranked by how much error each method typically introduces, best first:

Measuring a ring that already fits

A rigid object with a fixed size. No tension error, no compression, no temperature effect. How to measure a ring

A plastic strip sizer

Behaves like a real ring, including testing whether it clears the knuckle. Cheap, and often free from a jeweller.

A paper strip

Does not stretch and spreads pressure across a wider contact patch. Everything string does, without the two failure modes. The full method

String or thread

Stretches, and cuts into the skin. Use it only when nothing else is to hand, and expect to be reading small.

Where string is actually fine

There is one job string does well: getting a rough bracket when you have nothing else and you only need to know whether someone is nearer a US 6 or a US 9. For narrowing down a gift purchase before you find a better method, that is genuinely useful.

It is not adequate for an engagement ring, a wedding band, or anything set with stones all the way round, where resizing may be limited or impossible. What can and cannot be resized later.