I have a small child road bike frame that takes 20 inch wheels. It comes with no information from the manufacturer, but from what I've learned there are two 20 inch wheel/tyre standards 406 and 451.
Based on measuring the fork, 451 wheels will fit but with very little clearance. If it were an adult road bike frame with 700/23c wheels/tyres I wouldn't be concerned but from what I've seen 20 inch tyres tend to be 'fatter' or rounder than their adult sized counterparts.
So - my question - is there a standard to define how high a tyre rises from a rim? If not, is there a calculation/rule of thumb you can use? Or even a crowdsourced resource of end users real world rim/tyre combinations?