Estimate thread used for a length of seam.
A lockstitch uses roughly 3\u00d7 the seam length in thread (needle + bobbin combined).
The amount of thread a seam uses is much more than the length of the seam, because the thread travels up and down through the fabric and around the bobbin thread in every stitch. This calculator gives an estimate from the total length of seams, which helps you decide how many spools to buy for a project.
Shorter stitches, thicker fabric, and extra rows of topstitching all use more thread, and serger or overlock seams use far more than a plain straight seam. It is better to buy a little extra, since a thread that runs out partway through a long seam can be hard to match from a new spool.
Use the thread weight and type suited to the fabric and keep the same brand and weight for the needle and bobbin where possible. Check how much thread is on each spool, since spool lengths vary widely between brands and sizes.
To find your own thread multiplier, wind a full bobbin, sew a measured length of seam, and then measure how much thread you used. Dividing by the seam length gives a figure that suits your machine, stitch length, and fabric.