: How to know after how much time will the toddler be x inches tall? The child is currently 2 feet 11 inches tall. I want to buy a balance bike for her which says the maximum inner leg length of the kid should be 18 inches.

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There's no way to know when a child will be a particular height, of course, other than waiting.

However, you can look at growth charts to see how children typically grow. There are two major sets that I'm aware of; the CDC growth charts and the WHO growth standards (I link to the CDC's page on the WHO growth charts, as I find it easier to navigate; they link towards the bottom to the WHO page itself.) In the US, the CDC recommends using the WHO standards up to 24 months old, and the CDC charts above that. CDC charts have the major weakness that they're based primarily on Americans, and so will have less utility in countries where growth is significantly different - either due to genetic causes or environmental ones.

Note that "Height" is sometimes called either "length" (for babies/toddlers) or "stature" (for older children). The CDC's charts use "Stature for Age" in the 2-20 year old child charts. Also verify you have the charts for the right sex - boys and girls grow differently, of course.

What you should do is find where your daughter has been on the last three or so measurements, in terms of percentile, on the length-for-age. My younger child (who will turn 3 in march), for example, was at (if I recall) about 60%, 75%, and 55%. Note the drastic differences - likely due to a combination of measurement error and different timings of growth spurts. That's why it's good to use multiple measurements rather than just the 'last' one.

From that range, find the two lines that best reflect the range. If three lines best reflect the range, use the outer two lines (ignoring the middle one). Then follow those lines until they meet the height you're looking at; mark the two points where the lines cross the height ("stature") line.

So, assuming my child will likely be between the 50th and 75th percentile, he will reach 41 inches (3'5") between 4 (when the 75th percentile line crosses the 41 inch line) and about 4.5 (when the 50th percentile line crosses the 41 inch line). That's relevant only for my son, of course; for your daughter you should look on your own.

If your daughter's last three measurements were in the 25%-50% range, as her current one looks to be (Assuming 35" was her measurement at 30 months), then it looks like she's likely to hit 41" between 4.5 and 5. However, if she actually measured more consistently between 50th and 75th percentiles (as you don't list any other measurements), she might be more likely to hit between 4 and 4.5. And again, remember these are not predictions, just standards; my youngest was in the 75th-85th percentile for his first year, regressing towards the mean slowly over the second year. This might, however, be enough for your current task (guessing how long a toy will be useful for).

Also remember; growth charts tend to smooth over things like growth spurts in unhelpful ways. Looking at the 2-20 chart for boys, the growth is basically a smooth up-sloping line implying a consistent growth per year from 2 all the way to 15 or 16; that's not because any individual boy grows consistently from year to year, but because the average is consistent. Children go through growth spurts at different times, and so the average appears smooth, even when the individual growth is not.