Andrew Mawson wrote:

Ibs per square foot

Oh so it is. I'd never twigged that very basic fact.

But for those who prefer less archaic units:

* ========================= *
* Lead sheet to BS 1178 *
* ========================= *
* Code Thickness Weight *
* No. mm kg/m^2 *
* ---- --------- ------ *
* 3 1.32 15.0 *
* 4 1.80 20.4 *
* 5 2.24 25.4 *
* 6 2.65 30.1 *
* 7 3.15 35.7 *
* 8 3.55 40.3 *
* ========================= *

--
Andy