From one of your rings, you can measure the internal diameter with preferably a caliper or a tool with enough precision then find the correspondence of the ring size in the table below:
Between 2 values, choose the higher value.

Or if you are not too far from Angoulême and the PIMAA shop, a ring case to measure the finger size and a “triboulet” to measure from a ring, are available in 21 rue de la cloche verte, 16000 Angoulême

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