A 1X2 market prices three outcomes, and draw no bet prices two of them after deleting the third. The conversion is two steps: remove the margin from the three-way market first, then renormalize home and away so they sum to one again. Do it in the wrong order and you price a market that never existed.
The two steps, worked
Take a match priced 2.10 home, 3.40 draw and 3.80 away. The implied probabilities sum to 103.35%, a 3.35% overround. Proportional removal, the rule the no-vig calculator applies, gives fair probabilities of 46.08%, 28.46% and 25.46%, fair prices of 2.17, 3.51 and 3.93, checked 2026-10-16.
Now delete the draw. The home and away probabilities no longer sum to one, so divide each by their combined total: 46.08% and 25.46% make 71.54% together, and the renormalized shares are 64.4% home and 35.6% away. Fair draw no bet prices: 1.55 home and 2.81 away.
Why the order matters
Removing the draw first and normalizing second bakes the draw's margin share into the remaining two prices, which overstates the home side and understates the away. The draw carried part of the book's margin, and that part has to leave with it. Margin removal per complete market, then renormalize: the same discipline the site's margin guides keep everywhere.
A quick check confirms the result: the DNB home fair price sits below the 1X2 home fair price, because without the draw your bet pushes on a tie, and 1.55 against 2.17 is exactly the direction a push option should move the price.
The comparison to the posted DNB price
The point of the computation is the comparison, not the number. If the book posts 1.50 on home draw no bet and your computed fair is 1.55, the posted price is shorter than fair, and the difference is what you are being charged for the convenience of skipping the third outcome. If the posted DNB price beats your fair price, something is mispriced, and the first suspect is your margin model, not the book's generosity.
When the shortcut tempts you
The tempting shortcut is dropping the draw from the raw prices and normalizing directly. Try it on the numbers above and the error is small per market and systematic: it always leans the same way, because the draw's margin share is not distributed randomly. Over a season of comparisons, the shortcut reads phantom value into every home favorite.
Remove the margin, drop the draw, renormalize. Two steps, and the two-way price is as honest as the three-way market it came from.