<!DOCTYPE html>
<meta charset="utf-8" />
<form id="form">
<input name="input" type="text" />
<select name="inunits">
<option value="0.001">мм</option>
<option value="0.01">см</option>
<option value="1">м</option>
</select>
<span>Перевести в:</span>
<select name="outunits">
<option value="1000">мм</option>
<option value="100">см</option>
<option value="1">м</option>
</select>
<input name="output" type="text" readonly="" />
<script>
with (form) {
onchange = oninput = function() {
output.value = input.value * inunits.value * outunits.value;
}
}
</script>
</form>