The Root Mean Square (R.M.S) value of a voltage or current is:
VRMS = VP / √ 2 and IRMS = IP / √ 2
where VRMS and IRMS are the R.M.S. values, and VP and IP are the peak values.
√ 2 ≈ 1.414
Similarly, multiply R.M.S. by √ 2 to get the peak value.
Values quoted are R.M.S. unless otherwise stated - normally the R.M.S. subscript is left off.
E.g. 240V A.C. mains has a peak value of 240 X √ 2 = 340V peak.