PHP IF/ELSEIF/ELSE issues

I'm trying to make a BMI calculator...so far, I have everything displaying properly, except for the actual BMI range (below, normal, above).

There's most likely an issue with my if/elseif statements, because the output is always wrong. Here's my code:

<?php
    $weight = $_POST['txtWeight'];
    $height = $_POST['txtHeight'];

    $selected_unit = $_POST['optUnit'];

    $metric_status = 'unchecked';
    $imperial_status = 'unchecked';

    $impCalc = ($weight * 703) / ($height * $height);
    $metCalc = $weight / ($height * $height);
    $below = 'below average.';
    $normal = 'normal.';
    $above = 'above average.';

    if (($impCalc < 18.5) || ($metCalc < 18.5))
        {
         $bmi = $below;
        }

    elseif (($impCalc >= 18.5 && $impCalc <= 24.9) || ($metCalc >= 18.5 || $metCalc <= 24.9))
        {
         $bmi = $normal;
        }

    else
        {
         $bmi = $above;
        }

    if (isset($_POST['btnEnter']))
        {
         $selected_unit = $_POST['optUnit'];
         if($selected_unit == 'imperial')
            {
             $imperial_status = 'checked';
             echo "A height of {$height} inches and a weight of {$weight} pounds = {$impCalc} BMI.  Your BMI calc is: {$bmi}";
            }
        else if($selected_unit =='metric')
            {
             $metric_status = 'checked';
             echo "A height of {$height} meters and a weight of {$weight} kilograms: {$metCalc} BMI.  Your BMI calc is: {$bmi}";
            }
        }


?>

What am I doing wrong?