[Nexus]

Попробуйте так:

document.addEventListener('DOMContentLoaded', function() {
    var dir,
        sqr = '²',
        mul = {
            '01': 100,
            '12': 10,
            '02': 1000,
            '10': .01,
            '21': .1,
            '20': .001
        },
        dim = [{
            unt: ' м',
            dec: 2,
            mul: 1
        }, {
            unt: ' см',
            dec: 0,
            mul: .0001
        }, {
            unt: ' мм',
            dec: 0,
            mul: .000001
        }],
        side = [].slice.call(document.querySelectorAll('input[type="text"]'));

    side.forEach(function(node) {
        node.addEventListener('keyup', function() {
            this.value = this.value.replace(/[,\.]+/, '.').replace(/[^\d.]/, '');

            var u = unit.filter(function(node) {
                return !!node.checked;
            }).shift().value || '';

            [].forEach.call(document.querySelectorAll('[data-area]'), function(node) {
                var group = node.dataset.area,
                    values = side.filter(function(node) {
                        return node.classList.contains(group);
                    }).map(function(e) {
                        return parseFloat(e.value) || 0;
                    }),
                    area = calculate(group, values);

                [].forEach.call(node.querySelectorAll('.area'), function(e, i) {
                    e.textContent = area[i] ? (area[i] * dim[u].mul).toFixed(2) + ' м' + sqr : '';
                });
            })
        });
    });

    var unit = [].slice.call(document.querySelectorAll('input[type="radio"]'));

    unit.forEach(function(node) {
        node.addEventListener('change', function() {
            var u = this.value,
                d = dir + u;

            [].forEach.call(document.querySelectorAll('.unit'), function(node) {
                node.textContent = dim[u].unt;
            });
            side.forEach(function(node) {
                var v = parseFloat(node.value);
                if (v)
                    node.value = (v * mul[d]).toFixed(dim[u].dec);
            });

            side.forEach(function(node) {
                node.dispatchEvent(new Event('keyup'));
            });
        });

        node.parentElement.addEventListener('mousedown', function() {
            dir = unit.filter(function(node) {
                return !!node.checked;
            }).shift().value || '';
        });
    });
});
if (document.readyState === 'complete')
    document.dispatchEvent(new Event('DOMContentLoaded'));