The behemoth Delta SkyMiles program contains many twists, turns and odd alleyways, none more confusing than the difference between the "Pay With Miles" and "Miles + Cash" programs.

Although they sound nearly identical, they work very differently. And to make matters worse, neither are the same as booking "award flights" using miles.

Here you'll find the basic differences between these options, how each one works and how you can determine which (if any) you should use to book your next flight with Delta.

The basics

In a nutshell, here are the differences between these different booking options:

"Pay With Miles"

Only available to those with a Delta co-branded credit card.

Lets you pay for some or all of your cash fare with SkyMiles.

Each SkyMile is valued at a set rate of 1 cent per mile.

You get to choose how many miles you use.

These tickets earn both Medallion Qualifying Miles and redeemable miles, but only on the portion of the ticket paid with cash.

"Miles + Cash"

Available to anyone with a SkyMiles account.

Lets you pay for some of your cash fare using SkyMiles.

The value per SkyMile varies from ticket to ticket.

Delta determines the split between cash and SkyMiles.

These tickets don't earn Medallion Qualifying Miles or redeemable miles.

Award flights are the normal way to use miles to book flights with Delta and its partners. Read our guide to booking Delta award flights for more details.

Generally, we recommend using the "Pay With Miles" feature if you loathe searching for award availability. We generally don't recommend using the "Miles + Cash" option unless you are trying to burn a very specific number of SkyMiles and have no other good options.

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Pay With Miles: How it works

Again, this option is available only if you have a Delta co-branded credit card attached to your SkyMiles account. Specifically, you must hold the Delta SkyMiles® Blue American Express Card.

To use Pay With Miles, search for a flight as you would normally. Don’t select "shop with miles" just yet. You should see a "Pay with Miles Eligible" tag at the bottom of each cash fare.

Select the fare as you usually would. On the next screen, you'll get a widget with a dropdown like this:

As you can see, you can offset the cost of the cash fare using SkyMiles in 5,000-mile increments.

SkyMiles are always worth 1 cent each when used in this way, which, while not spectacular, isn't far from our estimated value for SkyMiles of 1.2 cents each.

Given how much simpler this option is compared with normal award booking, those with little patience for blackout dates and other award calendar headaches might be willing to forego potentially higher value from their SkyMiles. However, it’s probably worth spending a few minutes comparing the value of the fares you find with "Pay With Miles" and normal award bookings (see the calculator below for help figuring this out).

Miles + Cash: How it works

Let’s say you want to use your SkyMiles to book a ticket, but you don’t have enough miles to cover the full cost. You can use the "Miles + Cash" option on most Delta-operated flights to do just that. Make a search in the Delta booking tool like normal, then look for the "Miles + Cash" option under "Show Price In."

Selecting this will switch the prices over to a combination of, well, SkyMiles and cash, like this:

Since Delta is setting both the split between the amount of SkyMiles and cash, as well as the value of the SkyMiles, determining the value of this option requires a little work. For example, the same ticket paying only cash costs $466.20 in this case (note that the Miles + Cash price above is for a “Main” fare):

Subtracting the cash portion of the Cash + Miles price ($191.20) from the full cash price ($466.20) determines how much cash is being offset by miles ($466.20 - $191.20 = $275). So $275 is being offset by the 26,500 SkyMiles.

You can use this calculator to determine whether this is a good deal or not:

If the value of 26,500 SkyMiles is more than $275, this is not a good deal. If they are roughly the same, it’s a wash.

Indeed, for the most part, these Miles + Cash bookings should be considered only as a last-ditch option for burning some extra miles if you don’t have enough for a full ticket.

The bottom line

They may sound similar, but Delta’s "Pay with Miles" and "Miles + Cash" options offer fundamentally different value propositions. Neither offers particularly good bang for your SkyMiles in terms of value, but the "Pay with Miles" benefit for Delta cardholders can make sense in some cases, where the "Miles + Cash" offset rarely does.