Entonces sera algo asi:
Código HTML:
Ver originalfunction calcular_imc(){
var peso = document.getElementById('peso').value;
var talla = document.getElementById('talla').value;
talla = talla / 100;
var imc = peso / (talla * talla);
imc = Math.round(imc * 100) / 100
document.getElementById('imc').value = imc;
}
<body onload="calcular_imc()"> Peso
<input type='number' name='peso' min='1' max='250'value =70 id='peso' onchange="calcular_imc()" required>
Talla
<input type='number' name='talla' min='100' max='250' value =170 id='talla' onchange="calcular_imc()" ><br required>
<?php
?>
<p align ='left'>Edad
<input type='number' name='edad' min='15' max='99' value ="edad" required> <?php
?>
IMC
<input type='text' name='imc' min='10' max='50' value ="" id='imc'><br></p>