There are many reasons why you might be drawn to a tankini swimsuit, from not having to worry about fitting the length, to the ability to mix and match bottoms, to just plain liking the style. It’s actually remarkably easy to turn a swimsuit pattern into a tankini – read on to learn how!
Front: Starting with your swimsuit pattern, draw a line across the front of the suit where you want the bottom of the tankini to be. (Remember that it will end ~1/4 shorter than your line because you need to hem the suit). A good rule of thumb is to slice across where the suit cuts up to go over your hips, but it’s up to you if your personal preference is shorter or longer. You probably want a slight curve to the hem line.
Back: Draw a line across the back of your suit that matches the line across the front. Check where they meet at the side and measure, to be sure the front and back will be the same length. If your suit is just a front and back, you could measure from the top down. For the Nautilus Swimsuit, I measured from the top of the Bottom Front and the Bottom Back. Again, you probably want a slight curve to the hem line.
Ease: To prevent the tankini from riding up, you want to add in a bit more ease below the bust (i.e. to the waist and high hip) than you want with a one piece suit. For a good fit, you want to make sure that you don’t alter the fit through the bust. For a swimsuit cut in one piece, you can do this by removing any shaping at the waist and flaring out the side seam of the suit from below the bust to the bottom.
To add ease in to the Nautilus suit in the front, cut a vertical line on either side of the center front notches (you don’t want to disturb those notches) and spread the pattern by 1/2″ on each line. Ease the extra fabric into the seam of the top piece as you sew. Remove the shaping at the waist from the pattern piece.
To add ease in to the Nautilus suit in the back, cut a vertical line at the center back and spread the pattern by 1-2″. Use the same length of elastic as called for in the instructions. Remove the shaping at the waist from the pattern piece.
Hemming: There are different ways that you can hem the bottom of your tankini. The simplest is just to fold the fabric up 1/4″ and zig-zag it in place (treating the main fabric and lining as one). If you feel confident in your elastic applying abilities and you want a bit of structure to the bottom, you can apply elastic to the bottom and fold it to the wrong side as you do to finish all the other edges of your suit, making sure that you don’t stretch the elastic as you sew. The biggest thing you need to pay attention to is how much the bottom of the tankini stretches when you take the suit on and off – make sure that the stitch you use can handle the level of stretch and if it can’t, use elastic.