The short answer is: for most grassy ground installations, dig your concrete base at least 18 to 24 inches deep. If you live in a cold climate with real winters, don't stop there. Check your local frost line and get the bottom of the footing below it, usually 30 to 36 inches deep or more in northern regions. That depth stops frost heave from slowly pushing the bench legs out of level.
Why depth matters
A park bench on grass looks simple, but it takes a lot of lateral load when people sit, lean, and shift around. A shallow pad can rock, settle, or tilt after one rainy season. In freeze-thaw climates, water in the soil expands as it freezes and can lift shallow footings. Deep footings anchor the bench into stable soil below the frost zone, which keeps the bench level and safe for years.
What I usually recommend
For a standard 6-foot park bench with two legs, I use two concrete footings. Each footing should be about 12 to 16 inches in diameter and at least 24 inches deep. In cold climates, extend that to 30 to 36 inches deep or below the frost line. In sandy or well-drained soil, 18 to 24 inches can work if you compact the bottom well and add gravel. In clay or wet soil, go deeper and add a 4 to 6 inch gravel drainage layer under the concrete.
Step-by-step installation tips
First, mark the leg positions on the grass and remove a square of turf slightly bigger than each hole. Next, dig straight-sided holes with a post-hole digger or auger. The diameter should be roughly three times the width of the bench leg or anchor bolt group. Add 4 to 6 inches of compacted gravel at the bottom for drainage. Then place a form tube or pour directly against the soil if the sides are clean and firm.
Set anchor bolts or embedded post brackets in the wet concrete, using a template to match the bench foot spacing exactly. Slope the top of the concrete slightly away from the anchor so rainwater drains off instead of pooling around the bolts. Let the concrete cure for at least 48 to 72 hours before bolting the bench down.
Alternatives to poured concrete
If you cannot pour concrete, precast concrete deck blocks or screw-in ground anchors rated for benches can work on grassy ground. They are faster to install, but they are not as permanent in freeze-thaw clay or loose soil. For a truly stable public or backyard bench, poured footings are usually the better long-term choice.
Final tip
When in doubt, go deeper than the minimum. The extra bag of concrete is cheap compared with fixing a leaning bench later. A properly installed base will keep your park bench solid, level, and safe through every season.