<table>
        <tr>
            <th rowspan="3">Inputs:</th>
            <td>Hole dia</td>
            <td><input id="dia" onkeyup="calculate()" type="text" value="18" name="dia"></td>
            <td><select id="diaUnits" onchange="calculate()" name="diaUnits">
                <option value="1">mil</option>
                <option value="39.37" selected>mm</option>
                <option value="0.03937">um</option>
            </select></td>
        </tr>
        <tr>
            <td>Plating thickness</td>
            <td><input id="thickness" onkeyup="calculate()" type="text" value="1" name="thickness"></td>
            <td><select id="thicknessUnits" onchange="calculate()" name="thicknessUnits">
                <option value="1">mil</option>
                <option value="39.37" selected>mm</option>
                <option value="0.03937">um</option>
            </select></td>
        </tr>
        <tr>
            <td>Via length</td>
            <td><input id="length" onkeyup="calculate()" type="text" value="60" name="length"></td>
            <td><select id="lengthUnits" onchange="calculate()" name="lengthUnits">
                <option value="1">mil</option>
                <option value="39.37" selected>mm</option>
                <option value="0.03937">um</option>
            </select></td>
        </tr>
        <tr>
            <th rowspan="2">Additional Inputs:</th>
            <td>Applied current</td>
            <td><input id="current" onkeyup="calculate()" type="text" value="1" name="current"></td>
            <td>A</td>
        </tr>
        <tr>
            <td>Plating resistivity</td>
            <td><input id="resistivity" onkeyup="calculate()" type="text"  value="1.9e-6" name="resistivity"></td>
            <td>Ohm×cm</td>
        </tr>
        <tr>
            <th rowspan="5">Results:</th>
            <td>Resistance</td>
            <td><input id="resistance" onkeyup="calculate()" type="text" name="resistance"></td>
            <td>Ohm</td>
        </tr>
        <tr>
            <td>Voltage drop</td>
            <td><input id="voltage drop" onkeyup="calculate()" type="text" name="voltage drop"></td>
            <td>V</td>
        </tr>
        <tr>
            <td>Power loss</td>
            <td><input id="power loss" onkeyup="calculate()" type="text" name="power loss"></td>
            <td>W</td>
        </tr>
        <tr>
            <td>Estimated ampacity</td>
            <td><input id="ampacity" onkeyup="calculate()" type="text" name="ampacity"></td>
            <td>A</td>
        </tr>
        <tr>
            <td>Thermal resistance</td>
            <td><input id="thermal resistance" onkeyup="calculate()" type="text" name="thermal resistance"></td>
            <td>°C/W</td>
        </tr>
    </table>

    <script language="javascript">
        function calculate() {
            // Getting input
            dia = document.getElementById("dia").value;
            thickness = document.getElementById("thickness").value;
            length = document.getElementById("length").value;
            current = document.getElementById("current").value;
            resistivity = document.getElementById("resistivity").value * 1000 / 2.54;

            // Getting units of input
            dia_unit = document.getElementById("diaUnits").value;
            thickness_unit = document.getElementById("thicknessUnits").value;
            length_unit = document.getElementById("lengthUnits").value;

            // Converting input to heterogeneous units, see http://www.wolframalpha.com/input/?i=um+to+mil;+mm+to+mil for details
            dia *= dia_unit;
            thickness *= thickness_unit;
            length *= length_unit;

            // Calculating
            rhoth = 98;
            area = Math.PI * (dia + thickness) * thickness;
            resistance = resistivity *  length / area;
            thermal_resistance = rhoth * length / area;
            voltage_drop = current * resistance;
            power_loss = current * voltage_drop;

            // IPC-2221 external layers constants
            k = 0.048;
            b = 0.44;
            c = 0.725;
            delta_t = 10;

            ampacity = k * Math.pow(delta_t, b) * Math.pow(area, c);

            // Output
            document.getElementById("resistance").value = resistance.toPrecision(3);
            document.getElementById("voltage drop").value = voltage_drop.toPrecision(3);
            document.getElementById("power loss").value = power_loss.toPrecision(3);
            document.getElementById("thermal resistance").value = thermal_resistance.toPrecision(3);
            document.getElementById("ampacity").value = ampacity.toPrecision(3);
        }

        calculate();
    </script>