At the moment, I have a 52/39 12/25 gear ratio on the road bike.
I shall be getting a new bike shortly that has a compact 50/34 on the front, but I am unsure what ratio cassette to get??
At the moment the climbs here in the S.Wales valleys are not too bad on what I am using. What ratio on the back would be best suited, that will not make the climbs harder or cause me to spin out any quicker?

Thanks in advance.

Go for 11 smallest to compensate for loss of the 52.
If you’ve dropped from 39 to 34 then to go from 25 to 23 or even 21, personally I like the closer ratios of 11-23 (and 12-27 on my XC bikes). With a compact you should be fine with 11-21.

I would say with the 34 inner ring, hill climbing will be easier than 39 with the same cassette, and with 50/12 you probably wouldn’ spin out either.
Try first with same cassette then, if needed an change to an 11/28 or 32 depending how you struggle on the hills.

34×21 only gives you a gear in the low 40s it is similar to your current gearing but I would go with a 25 on the back as a bit of insurance against being knackered.

i would say change the cranks! however 11-23 at the back should makeup most of the difference.

In my experience going compact makes you lazy……

A 50/34 – 11/25 should suit you fine. I use that on my roadbike and it works for me, although think i might move to a 12/28 or 12/25 as i rarely use the 11

If you live in S Wales go for 13 -29 with a compact, cruise up the hills and freewheel above 35mph down other side; and if you venture out into mid Wales area you`ll be glad of the 29 at times 🙂
Aside from that I manage mid Wales with above in spring / autumn, or in summer 12-25

Thanks fellas. 11/23 looks the favourite then.
As said. Do not want to struggle any more on the climbs, also not want to spin out too quickly…

Personally I’d go 12-23 but everyone is different. I like compact but have changed the 34 inner to a 36. What you need to do is search for gear calculator and put in some ratios and see how they compare.

39/25 is the same as 34/22.

The jump from 34 to 50 would irritate me, but then so does 39 to 53.