TTC, irregular cycles, how to tell when I'm ovulating?

Hey :) my partner and I are TTC, so for the last 4 months since we decided to try I have recorded my cycles, which I have never done before. I have realised my cycles are irregular, the last four being 18, 35, 30, and 24 days long. We have been trying for 3 months and I'm not pregnant, so I'm worried we are not calculating the ovulation date right, or wont be able to because my cycles are irregular. Do ovulation test kits work if you dont have regular cycles? Is there any ways of pinpointing ovulation with irregular cycles? Many thanks for your help :)