Yarn held together calculator
Enter the length and net mass on each yarn label. When strands are held together, their mass per metre adds; the calculator shows the resulting length per 100 g and how far the shortest package can run.
Why two 400 m / 100 g strands become 200 m / 100 g
Each strand uses 25 g over 100 m. Held together, the same 100 m of fabric uses 50 g. That makes 200 m per 100 g, with at most 400 m of two-strand running length from one package of each. It does not make 800 m of combined yarn.
Use the result to plan the fabric
Length per 100 g compares linear density; it does not tell you the stitch gauge, fabric feel or exact weight-category label. Make and wash a swatch with both strands and your chosen tool. Then check the measured gauge or compare it with a pattern yarn.