How do you know how much dirt is neede.

Have a landscape person come look then you could get like a yard or 1/4 yard he will tell you what you need and deliver it.

Only you can determine that just keep filling and tamping it down til level. Picture may help?? What is dip from?water flowing or a sink hole of sorts?

You can calculate it buy buying 2-100' rope or twine bundles from the dollar store, 4-6 stakes and a long level.

Stake out the area you want to fill. Tie the ropes twine from stake to stake and put the level on the ropes/twine, close to the ground. You should have a 2-4% slope anyway, but getting the twine "level" will be close enough.

Then measure the depth between the top of the twine/ropes at various points.

Average the sum of the various depth measurements.

Now you have Width x Length x Height (Depth) and can calculate how many Cubic Feet of soil you need. Voila'.

If you're going to have it delivered and dumped into your yard, then you'll be paying for a trip charge no matter what and I would round-up on my calculations. Better to be 10% over than 10% under.

Be sure you compact the soil as you go. Ideally, this should be done mechanically, but hose water will help too.

And make sure the yard isn't sinking for a bad reason, like failed sewer line or seismic activity, before you start. :-)

https://homeguides.sfgate.com/calculate-much-dirt-need-fill-spot-lawn-96208.html#:~:text=You%20can%20calculate%20how%20much%20dirt%20you%27ll%20need,is%20typically%20sold.%20What%20Is%20a%20Dirt%20Calculator%3F

Measure both the width and length you're looking for and multiply those two numbers together - that will give you the square footage. Now determine how deep you want it. With that information the people you order the dirt from will be able to help you determine how much you will need.