I went to wikipedia and found these:
HRmax = 220 ? age (caution: can vary significantly!)
This is attributed to various sources, often "Fox and Haskell". While the most common (and easy to remember and calculate), this particular formula is not considered by some to be a good predictor of HRmax.
A 2002 study [4] of 43 different formulae for HRmax (including the one above) concluded the following:
1) No "acceptable" formula currently existed, (they used the term "acceptable" to mean acceptable for both prediction of V_{\mathrm{O}_2 max}, and prescription of exercise training HR ranges)
2) The most accurate formula of those examined was:
HRmax = 205.8 ? (0.685 * age)
This was found to have a Standard Deviation that, although large (6.4 bpm), was still deemed to be acceptable for the use of prescribing exercise training HR ranges.
Other often cited formulae are:
HRmax = 206.3 ? (0.711 × age)
(Often attributed to "Londeree and Moeschberger from the University of Missouri–Columbia")
HRmax = 217 ? (0.85 × age)
(Often attributed to "Miller et al. from Indiana University")
They results ranged from about 283 to 289.
I am measure my heart rate with a HRM, The forerunner 305 (which is pretty cool i might add)