A simple calculator and instructions for the ubiquitous beginner MYOG sewing project.
Fabric width (zipper edge):
Fabric height:
Litres:
Fun fact: this is a minimum volume estimate that doesn't account for bulging. Big manufacturers fill bags with small beans or balls of known volume to accurately measure volume.
Recommended seam allowances for your chosen measurement unit:
| Inches | Centimeters |
|---|---|
| 0.25 (1/4) | |
| 0.375 (3/8) | 1 |
| 0.5 (1/2) |
A #5 zipper sewn with a ~ 5/16 inch (8mm, quarter of the zip tape width) seam allowance does not need a zipper seam allowance, as the panel length will remain the same. If using different zipper sizes or sewing with different seam allowances, you can change the zip seam allowance box, although the difference in volume and dimensions when using a different zipper size is negligible.
Sewing Instructions
Sew a zipper half to each half along the long edge by first sewing a zipper upside down, then folding it over on this stitch line and top stitching it down.
If using a waterproof zip, upside down means shiny tape touching the fabric, and teeth pointing upwards. For a normal coil zipper upside down means the teeth touch the fabric, and the smooth tape is facing up.
Basting stitch with 1/4 inch seam allowance or less a folded over piece of 2.5 to 3 inch grosgrain at the midpoint to form the pull tabs/attachment loops on the zipper.
Fold each side inwards to join in the middle of the fabric
Insert your zipper slider. Make sure it is upside down, as you are now working with an inside out bag. If only using one slider, pull it all the way to fully close the zipper, then re-insert it a second time, leaving it halfway across the zip.
If adding two sliders, insert the second from the opposite edge of the zipper tape so they join and close the zipper in the middle.
Sew the two sides with your chosen seam allowance, going back and forth over the zipper tape for reinforcement..
Before sewing the second side, reach through and partially open the zipper. This will make fully opening it later to invert the bag the right way out much easier.
If your fabric choice isn’t too stiff, you can fold the seam allowance back on itself and stitch it down to hide the raw edges. Alternatively you could bind the raw edges with folded over grosgrain ribbon. I chose to fold back the seam allowance and sew it flat for the following photos.
Using a pencil, pen or something that won't soak through your fabric onto the right side, lightly mark out 4 squares at each corner of the fabric, between the two side stitches, where the square sides are half the desired bag height. You can extend the lines into the seam allowance if you like, but the half depth sized squares must be outside the seam allowance.
Repeat this on both sides of the fabric.
Pull and pinch the bag into a 3D shape, forming triangles in all 4 corners using the lines you just marked out to guide the shaping of the bag.
Sew along all 4 of these marked lines.
If your bag is quite big, you can snip off some of the excess triangle, but make sure to leave around 1/2 inch seam allowance, and give an extra row of stitching to minimise any chance of the snipped off thread unravelling long term.
Invert your bag, you are finished!
