<script type="text/javascript">
    function check_digits(if_no_points) {
        var key = event.keyCode;
        if (!(key >= 48 && key <= 57) && key != 8 && key != 46) { event.returnValue = false; }
    }
    function calculate() {
        var price = 0;
        var room_length = document.calc.room_length.value;
        var room_width = document.calc.room_width.value;
        var tubes_quantity = document.calc.tubes_quantity.value;
        var lightpoints_quantity = document.calc.lightpoints_quantity.value;
        var angles_quantity = document.calc.angles_quantity.value;

        if (room_length > 0 && room_width > 0) {
            document.getElementById('roof_size').innerText = room_length * room_width;
            document.getElementById('roof_perimeter').innerText = 2 * room_length + 2 * room_width;
            if (angles_quantity >= 4)
                price = room_length * room_width * 450 + (2 * room_length + 2 * room_width) * 50 + tubes_quantity * 300 + lightpoints_quantity * 200 + (angles_quantity - 4) * 100;
        }
        else
            document.getElementById('roof_size').innerText = "0";

        document.getElementById('price').innerText = price + "руб.";
        return true;
    }
</script>
<form name="calc">
    <table>
    <tr>
        <td>Длина комнаты (метры):</td>
        <td><input size="10" name="room_length" onkeypress="check_digits()" onkeyup="calculate();" /></td>
    </tr>
    <tr>
        <td>Ширина комнаты (метры):</td>
        <td><input size="10" name="room_width" onkeypress="check_digits()" onkeyup="calculate();" value="" /></td>
    </tr>
    <tr>
        <td>Количество труб:</td>
        <td><input size="10" name="tubes_quantity" onkeypress="check_digits()" onkeyup="calculate();" value="" /></td>
    </tr>
    <tr>
        <td>Кол-во точек освещения:</td>
        <td><input size="10" name="lightpoints_quantity" onkeypress="check_digits()" onkeyup="calculate();" value="" /></td>
    </tr>
    <tr>
        <td>Кол-во углов в комнате:</td>
        <td><input size="10" name="angles_quantity" onkeypress="check_digits()" onkeyup="calculate();" value="4" /></td>
    </tr>
    <tr>
        <td>Площадь потолка:</td>
        <td><span id="roof_size"></span> м²</td>
    </tr>
    <tr>
        <td>Периметр потолка:</td>
        <td><span id="roof_perimeter"></span> м</td>
    </tr>
    <tr>
        <td>Стоимость потолка под ключ:</td>
        <td><span id="price"></span></td>
    </tr>
    </table>
</form>