There's no fixed "ounces per 5 gallons" answer, because the same weight of hops contributes wildly different bitterness depending on alpha acid, boil time and boil gravity — a rule of thumb that ignores those three variables will be wrong for most real recipes.
A representative single-addition example
1.5 oz (42 g) of a moderate 6.5% alpha hop, boiled the full 60 minutes in a 19 L (5 gal) batch at 1.052 boil gravity, computes to 32.6 IBU by Tinseth or 43.8 IBU by Rager — a reasonable single-addition bittering charge for a pale ale or standard IPA. Run this exact addition.
Working backward from a target instead
The more useful question is usually "how much hops do I need for a target IBU," which depends on three things you should check before reaching for any amount: your target IBU (set by the style you're brewing — see the style reference), your hop's actual alpha acid (check the hop reference for a typical range, or better, your bag's actual COA), and your batch's boil gravity (higher-gravity boils extract proportionally less bitterness from the same weight — see why do my IBUs differ between calculators for how much this moves the result). Rather than solving that algebra by hand, enter a starting weight into the IBU calculator and adjust it until the computed IBU matches your target — it's faster and less error-prone than working the formula backward yourself.
Bittering vs. flavour vs. aroma additions
"How much hops" also depends heavily on what job the hops are doing. A bittering charge early in the boil (45-60 minutes) needs relatively little weight to hit a meaningful IBU target, because a full boil gives near-maximum utilization. A late flavour addition (10-20 minutes) contributes noticeably less bitterness per gram — see how many IBU is 1 oz of Cascade at 60 minutes for exactly how much boil time alone changes the number from an identical addition. Aroma and dry-hop additions contribute essentially no measured IBU at all regardless of weight — see does dry hopping add IBU — so a hop-forward modern IPA can use several ounces of dry hops per 5-gallon batch for aroma while its actual bittering charge stays comparatively modest.
The one number that doesn't change with any of this
Batch volume itself matters as much as any hop quantity — the exact same weight, alpha acid and boil time in a 10-gallon batch instead of 5 contributes roughly half the IBU, because bitterness is a concentration across the finished volume, not a fixed amount per addition. Always double-check your actual post-boil volume matches what you entered into the calculator before trusting a computed target.