Don't know what car you have.

If the centre is held in with bolts, it's an H165 1200 Ute Diff. Can be replaced with Bluebird centre or another 1200 Ute centre.
If the centre is held in with nuts, with studs coming out of the diff housing, then it's an H145, which is one of the most common diffs of all.

If it's got a plate on the back, it's a 120Y borgwarner.

As the H145, H165 are a contained carrier, there is no measurement required, just pull the axles, replace the centre, and reinstall the axles.