Getting from A to B - conversions and math

Fabric Calculator
An important note when you are calculating how much fabric: Remember what you are counting, threads per inch or stitches per inch. What do I mean? With evenweave and linen, you are often stitching over two threads. That means you need twice as much if you are counting threads which is how fabric is defined, instead of stitches which is what your chart describes.

Also remember that you need a margin on each side of your stitching, usually 3 inches for framing. Once you have determined the size of your pattern given the thread count of your fabric, add six inches to each dimension to get the three inch border. If you decide to use something different for your margin, make sure you write a note to remind yourself before you start stitching three inches down and three inches over.

Stitching The Night Away fabric calculator, you enter the particulars, it calculates the fabric size.
Project Calculator - you give the project dimensions in stitches, it determines size for various thread counts from AAN.
Cyber Stitchers online floss conversions, fabric calculator and free graph paper:
Yarntree's fabric calculator
Stitches to Inches chart (pdf to download and take with you) from yiotas-xstitch

Fabric to Fabric
Descriptions and thread counts for many brands of fabric (oslo and ariosa for example) at Nordic Needle
Fabric Descriptions, sizes, and DMC equivalents at 123 Stitch
Kathy Dyer Fabric name, description and manufacturer

Fiber Conversion
The issue with this information is more than just accuracy, which ultimately is a matter of individual judgement. Fiber manufacturers change, add and delete colors all the time. So you may be looking for a substitution for an old DMC color no longer made, or for thread no longer manufactured.

Speaking of fiber conversion, how about making perle cotton out of floss? No really! Linda Kay

Kathy Dyer has the most comprehensive list of resources, however the currency is often dependent on someone else's contribution. After all, these pages have been online for a very long time. Cameo Rose also has a lot of information. Some of the pages have not been updated in years, however. You may need to check multiple resources to find the particular conversion you are looking for.

Find Stitch has an extensive list of resources for thread conversion as well as color charts for DMC, Anchor, Madeira and JP Coates. Find Stitch

My suggestion if you are using something other than the original threads, take all of them together and place on a sheet of white paper under good light, and make sure they go with each other. Then take the whole set and spread it out on your fabric, making sure you don't have something clashing there. It's your project, so you are the person who needs to be happy with the colors.

Cameo Rose's links to all kinds of conversion tools for silk, beads, yarn, fabric, cotton including Weeks and GAST
Kathy Dyer's FAQ page with conversion charts and explanations of DMC color changes over the last decade or so. She also has Mill Hill to DMC and Anchor, the old JP Coates colors, and color descriptions for Kreinik metallics
DMC to Mill Hill at Wichelt
Designer Forum Links for fiber conversions
AAN guide to various floss organized by color family with color swatches
Fiber Images tables for DMC to Anchor, Coates, Madeira and Anchor to DMC
Fiber (and bead) conversion database for floss (cotton, silk, rayon), flower thread, needlepoint yarn

Willow Fabrics:
DMC to Anchor by number with color description
DMC Light Effects to Kreinik braid by color (with some old DMC colors too)
DMC to Soie D'Alger silk
Also see Cyber Stitches above.