Actually, mIRC seems to be capable of only 19 digits for whole numbers, and 20 digits (including the '.') for floating points, but only up to 6 decimal positions.
9999999999999999999
999999999999999999.9
99999999999999999.99
9999999999999999.999
9999999999999999.999
999999999999999.9999
99999999999999.99999
9999999999999.999999
Beyond these, it starts rounding dropping the lest significant number.
- Raccoon