Using include from a different folder

i have a project using php; i have an include folder where i have my header.php and footer.php my style sheet and others are linked in the head tag of the header.php

i have another folder named calculator, now i include the header.php and footer.php in the index file of the calculator folder and it works but the css does not take effect

Here is the code in the calculator index file

<?php
    include '../include/header.php';
?>

    <form action="" method="post" enctype="multipart/form-data">
        <div class="form-group">
            <label for="exampleFormControlInput1">Name of Share</label>
            <input type="text" class="form-control" id="exampleFormControlInput1">
        </div>
        <div class="form-group">
            <label for="exampleFormControlInput1">Number of Shares</label>
            <input type="text" class="form-control" id="exampleFormControlInput1">
        </div>
        <div class="form-group">
            <label for="exampleFormControlInput1">Purchase Price</label>
            <input type="text" class="form-control" id="exampleFormControlInput1">
        </div>
        <button type="submit" class="btn btn-primary">Submit</button>
    </form>


    <?php
    include '../include/footer.php';
    ?>

please advise