Concrete Footing Depth Calculator
Estimate bottom-of-footing depth from frost line, soil bearing, support load, width, cover, embedment, uplift resistance, sliding resistance, and excavation allowance.
Footing Depth Result
| Footing case | Typical depth driver | Common bottom depth | Watch point |
|---|---|---|---|
| Deck post pier | Frost line and uplift | 36 to 48 in | Use full embedment below grade |
| Garage stem wall | Local frost map | 36 to 54 in | Top cover must fit wall stem |
| Shed thickened edge | Cover and bearing | 12 to 24 in | Frost rules vary by use |
| Retaining wall footing | Sliding and bearing | 24 to 48 in | Check toe pressure separately |
| Soil profile | Allowable bearing | Friction range | Depth behavior |
|---|---|---|---|
| Silt or soft clay | 1000 psf | 0.25 to 0.30 | Needs wider footing |
| Firm clay | 1500 psf | 0.30 to 0.40 | Common residential base |
| Dense sand or gravel | 2500 psf | 0.45 to 0.55 | Good sliding resistance |
| Weathered rock | 4000 psf | 0.55 to 0.65 | Often controlled by cover |
| Check | Formula used | Pass target | Design response |
|---|---|---|---|
| Frost | Frost depth plus allowance | Bottom below line | Lower footing bottom |
| Bearing | Total load divided by area | Pressure below allowable | Increase width or length |
| Uplift | Dead weight divided by uplift | FS 1.5 or better | Add cover, mass, or anchors |
| Sliding | Friction plus passive soil | FS 1.5 or better | Add embedment or key |
| Preset | Support | Load basis | Primary warning |
|---|---|---|---|
| Porch column pad | Isolated pad | Point load | Check roof uplift |
| Basement wall strip | Continuous wall | Line load | Frost depth controls |
| Pergola uplift pier | Post pier | Point plus uplift | Dead weight may be low |
| Shop column pad | Isolated pad | Heavy point load | Bearing area controls |
When digging a hole, if you ask most folks, they’ll tell you it’s about how hard you have to hit the dirt with shovel. But in reality, it’s all about what the dirt does when weather changes. Whether it’s shifting or freezing, a concrete footing anchors your house or deck in place. When that concrete cures, it become very difficult to dig out again without breaking up and restarting. That means depth matter and getting it incorrect can be quite expensive.
With this tool, you type in your local conditions and the calculator do the rest of math for you. You do not need to guess, convert, or calculate coefficients.
How to Use the Footing Calculator
First thing: What about the frost line? It’s not a recommendation; it’s a hard boundary. Water in soil expands when it become ice, and if your footing extend above the frost line, your footing rise up. Then comes spring thaw time, and soil settles again. Now your house is lighter than the surrounding earth so it’ll settle lower than everyone else’s. This results in cracked windows and sloping floor.
The footing calculator factors your entered frost-depth number so it will always be below the freeze-out area. Add a couple of inches for good measure. This is typicaly the case. Remember, frost lines aren’t exact points marked on map but are areas that shift from year to year.
But you also has to think about type of soil. Sand is dense and can holds a fair bit of weight before compacting. Clay is soft and compacts under load. To compensate for that, your footings must be wider so that the weight spread across more area. That’s why the tool compare the amount of weight on you vertically with the maximum bearing capacity of whatever soil type you choose. It’ll tell you if the load are too heavy. Digging deeper won’t help you fix a bearing failure. Instead, you need to make footing wider. Width addresses the bearing problem. Depth resolves the frost problem.
Wind uplift is one of big issues for lighter structures such as pergolas or deck posts. Uplift is where wind pushes up against your siding which acts as a lever trying to remove your posts from the earth. You need a large enough footing to prevent it from pulling out. The footing calculator calculates an uplift factor of safety. Below 1.5 and you are probably going to pop out in a good sized storm. The solution is to add mass (weight) or increase the soil cover above footing. Use versus weight.
But what about sliding? That’s an issue especially with retaining walls. Soils pushes laterally (sideways) against the wall, while gravity wants to pull down. Friction between base of the footing and the soil keeps it in place, depending on soil type. This tool provides a friction coefficient based off your soil type and tells you whether it will slip. If so: you’ll either require a wider footing or use a key.
Typical footings for a shed, garage stem wall, etc., can be found on the page in the reference tables. These should of been used as a guideline, but your site conditions always come first. Don’t skip the excavation part. What depth says the drawing? Nope, that’s not what depth you dig. Leave yourself plenty of room to work and drain with gravel. A good solid base and gravel will allow the concrete to sit above any standing water. Don’t let it cure wet. Don’t let it get soft. Give it some support and keep it dry.
The calculations is for preliminary sizing. This is intended as a starting point for sizing things out. It helps you understand how big something might be. But then again, it doesn’t. Local codes has the final say. The frost map is different, the soil profile is different. There may be expansive soils or seismic risk. These can only be vetted by a trained expert. Like a moddern pro engineer.
Use this to decide where to dig. Your local rules will dictate what’s permitted.
