How deep do I need to make the post footings for a residential deck in Northern Michigan? The 12" X 20" deck will be less than 2 feet off the ground.
The post depth depends on the frost line. The size of the footing depends on the soil type under the footing.
You need to dig until at least under the frost line, and then, if you are not on rock or undisturbed/compacted soil, dig until you get there. You can analyze the soil as you dig. The size of the pad of your footing depends on the soil: gravel, sand, or clay. See also https://www.decks.com/how-to/articles/soil-conditions-for-deck-footings
Check with your local municipality, they have this info for you.
Here's a map of the frost line (depths), and you'll see that Michigan is around 55".
Always check with your local building inspections department to see what is required in your area. Furthermore, you must install footings at least 12” below unsettled soil. If you are building into the side of a steep hill or beside a retaining wall you will have to maintain the minimum distance to grade not only vertically but also laterally. In order to achieve a suitable lateral barrier you may need to dig your foundation much deeper than on a flat surface.
Your best first resource would likely be the permitting requirements for your county and the county inspections office. Their website and their staff would provide the requirements as well as have the experience to guide you about the typical challenges and solutions given the climate, soils, and topography. In most jurisdictions a deck less than 30 to 36 inches off the ground does not require permitting. But the recommendation is to educate yourself on them anyways. It will help you to formulate a solid design.
Additional resources might be your local landscaping supplies distributor. They will have experience working with similar projects and understand the appropriate materials and methods to employ.
To fully address the question - and if by chance permitting were required (e.e. due to slopes) you would want a site and soils analysis by a geotech. That's probably not in your plans due to expense so you might do a little of the similar legwork they would do on your own. Do a few soil borings yourself to verify what you're building on top of. When you know what you're working with that will help to look online for the strength of that soil and also what kinds of piers are best to use.
As the commenter mentioned you will need to go fairly deep in any case just to drop below the frost line: so the depth is probably sufficient. You will likely have multiple options available for the types of footings to use: though the standard concrete piers you are likely considering would seem to suffice since you have not mentioned any sloping considerations.
It might be worth reconsidering if you even need to use cylindrical poured footings. Have you looked at the possibility of using Diamond Piers or Helical Piers? A lot of municipalities are starting to allow these alternative options, and there is a lot of evidence that they are just as effective as the traditional concrete footings. They cost a bit more, but the installation (I am assuming you are doing this yourself, or you wouldn't be asking the question) is far less labor-intensive.
Your typical 12" cylindrical footer will take at least 4 bags of concrete, without having the bottom belled out. Imagine trying to dig a hole for these suckers with a post hole digger:
No thank you.
This guy is installing Diamond Piers. He isn't even sweating:
If you are wondering, there is a good amount of evidence that these don't have issues with frost heave, which you might expect from a shallower footing.
Helical Piers are a third option. They may require some equipment to install, but can also be driven about as deep as you would ever want. They will support almost anything:
I personally would be more than happy spending a few hundred extra dollars on the total cost of a deck, if it meant that I wouldn't have to pour footings. If you decide to look into either of the Diamond or Helical piers, make sure to check with your city inspector first.