Long ago, I had a lovely Swedish Mauser, and needed some reload data. I had one of Steve Faber's early strain gauge systems, but the only data conveniently available used powders I didn't have and was in CUP. What to do? I collected a handful of SAAMI maximum pressure specifications for rifles that gave both the CUP and the PSI ratings and ran a regression on the data to get the formula that connects them. That didn't take long. My results appeared in Varmint Hunter not long after. Since I was only interested in rifle pressures, the data I used was rifle data, and the results only applied to rifles, not to handguns. Those interested in a more detailed explanation of what I did can read this: https://www.shootingsoftware.com/ftp/psicuparticle2.pdf A couple of years after the article appeared, I found that Dr. Lloyd Brownell had wondered about converting back and forth between the two systems, and had published a conversion curve decades before. The conversion has a little curvature in it, but over the range of about 35-65 KPSI, the simple straight line model works quite satisfactorily. I was able to choose a peak KPSI for my Mauser, and sometime after I did that, SAAMI arrived at a recommended peak very close to the one I chose. Nice. So for the simple model that applies only to centerfire rifles, the formula for ANSI specs is: PSI = -17,902 + 1.51 * CUP Today, I have much more data readily available and had a fit of ambition enough to run the model with handgun data included. The 357 Magnum was a major outlier, and was omitted. SAAMI specs have a large number of odd human choices in them, and that's apparently one. There are still a couple of strange examples in the data, but none so great as the 357. That left me with 38 data pairs, which is plenty. For those who want a model that covers all centerfire firearms, you can use this one: KPSI = 14.39 + .01716*KCUP^2 The raw data have significant random noise. Since the input data is imperfect, the output is not exact, but it is close enough for most practical purposes. Also, the exact results will vary a bit if you choose cartridges different from my selection. If you have a large batch of nearly identical cartridges, randomly select 10, measure their peak pressure, and then draw and test another 10, you'll practically always get two different numbers. You shouldn't expect modern test equipment to repeat much closer than about 1,000 PSI. Bear that in mind as you compare the model with actual data.