There's no single OG that produces 6% ABV — it depends on where fermentation actually finishes, and that's set by your yeast's attenuation and your mash temperature, not by the OG alone. What you can do is pick a realistic target FG first, then solve backward for the OG that pairs with it.
Two realistic starting points
If you expect a fairly dry finish around 1.012 FG (roughly what a clean American ale strain gives on a moderately fermentable wort), you need an OG of about 1.058 — run that pair and the calculator confirms 6.04% ABV by the simple formula, 6.22% by the advanced one. Check it here.
If you expect an even drier finish around 1.010 FG (a higher-attenuating strain, or a mash run on the cooler side for more fermentability), you only need an OG of about 1.056 for the same 6% target — run that pair and it confirms 6.04% ABV simple, 6.19% advanced. Check it here.
Notice both land at essentially the same simple-formula ABV (6.04%) despite different OG and FG — that's expected, since the simple formula only cares about the gap between OG and FG, not their absolute values. The advanced formula differs slightly more (6.22% vs. 6.19%) because it isn't a purely linear function of that gap.
Why you have to guess an FG first
ABV comes from the difference between OG and FG, not from OG alone — the same OG can produce very different ABV depending entirely on how far fermentation actually goes. Picking a target FG first, based on your yeast strain's published attenuation range and your planned mash temperature, is the only way to work this backward sensibly. Check your chosen strain's range on the yeast reference — a strain rated for 65-70% attenuation and one rated for 78-82% will need noticeably different OGs to land at the same 6% ABV target from the same recipe.
Getting there in practice
Once you've picked a target OG, hitting it is a grain-bill and efficiency question, not an ABV-calculator question — the strike water calculator handles the mash-temperature side of fermentability, and your recipe software or a manual gravity-points calculation handles how much grain gets you to that OG at your system's actual efficiency. If your OG comes in short of target on brew day, see my OG came in too low — how do I fix it rather than accepting a lower final ABV than planned.