If you're sitting on a healthy stash of transferable points and miles and want to fly United Airlines for less, your first instinct may be to move them to United's MileagePlus program . But thanks to Star Alliance partner Air Canada, you can often book the same United-operated flights using Air Canada Aeroplan points instead — and, in some situations, you'll pay fewer points than if you booked with United miles.

Aeroplan can be particularly useful if you earn American Express Membership Rewards points or Capital One miles since both transfer to Aeroplan but not directly to United MileagePlus.

Here's when it makes sense to use Aeroplan points to book United flights, how to make these bookings online and the best ways to earn Aeroplan points .

Related: Why Air Canada Aeroplan has become my favorite Star Alliance loyalty program

Both Aeroplan and MileagePlus now price United-operated award flights dynamically , meaning the number of points or miles required fluctuates depending on demand.

If you are considering whether to redeem United MileagePlus miles or Aeroplan points for your next United-operated flight, you can quickly and easily compare the prices of both at the same time using the handy Seats.aero tool .

Here are some occasions when it may be a better deal to book United flights with Aeroplan points than other points and miles, such as United's own MileagePlus miles:

Related: United unveils stunning next-generation Polaris cabins with caviar service and much more