You have your 20-leg itinerary planned, your next RTW is right around the corner or it's just your first longhaul return flight to a remote destination. Whatever the reasons, it's a lot of work to find out which loyalty programme to join to collect the most miles for your trip.
Accredited mileage will vary greatly from airline to airline, each booking class has their own percentage, fixed values or both. On top of that, one mile at one programme is by no means the same as one mile at another programme. Some don't even have "miles"..
To make the choice a little bit easier, bonusmiles.net tries to calculate how many qualifying status- as well as bonusmiles (or whatever the terminology may be) you will receive for a given itinerary.
All mileage projections are unofficial calculations only. They are no more than an educated guess and you should not rely on these results! After having compared them on bonusmiles.net, make sure to look up the actual tables at each respective programme before making a decision. (And feel free to let me know if the results are different!)
How to distinguish between operating and marketing carrier?
Currently there's no option to enter both carriers for a more correct calculation. If in doubt, enter operating carrier for Star Alliance flights and marketing carrier for OneWorld flights. I'll probably add a feature to enter both carriers in the future.
How is the departure date relevant?
While the departure date is optional, some programmes have blackout dates during which fares do not accrue any bonusmiles as well as special time periods in which they double the bonus etc. Therefore the departure date is required for a more correct projection of achievable bonusmiles.
The mileage should be different if I already have a status
Mile accruage often times doubles on condition of a preexisting status. Say you already have Gold at a programme, it's likely that the accrued status mileage for any given flight will be different to the amount you'd receive without any previous status. So far bonusmiles.net ignores this and calculates the mileage for users without a prior status. Most of the times, the bonus you'll receive will be as simple as doubling the amount, so the simplicity of the itineray-form is probably more important. This might change in the future though.