Very little. Roasted barley's Lovibond rating (typically 300-550°L, among the darkest grains in common use) means a small percentage of a grain bill contributes disproportionately more colour than the pale base malt making up the rest of it — this is the single most common surprise for a brewer building their first dark beer recipe.
Three real amounts, same base grain bill
Starting from 4.5 kg of pale base malt (2°L) in a 19 L batch, adding roasted barley (500°L) at three different weights:
| Roasted barley added | Resulting SRM |
|---|---|
| 0.5 lb (0.23 kg) | 23.2 |
| 1.0 lb (0.45 kg) | 35.8 |
| 1.5 lb (0.68 kg) | 47.1 |
Check the 1 lb example directly — notice that doubling the roasted barley from 0.5 lb to 1 lb doesn't simply double the SRM (23.2 to 35.8 is roughly 1.5x, not 2x), because the Morey equation's MCU^0.6859 exponent compresses the relationship between colour contribution and final SRM rather than scaling it linearly.
Where "black" actually starts
Most competition guidelines and common usage treat SRM in the mid-30s and above as effectively "black" — the 1 lb (0.45 kg) addition above already crosses that line at 35.8 SRM, using less than a pound of roasted barley against 4.5 kg (about 10 lb) of pale base malt. That's under 10% of the grain bill by weight (0.45 kg against 4.5 kg of base malt, roughly 9%) producing the entire visual difference between an amber beer and a black one. Push to 1.5 lb and the beer is unambiguously black at 47.1 SRM, with further additions moving the number without much additional visible change — the same diminishing-visual-returns effect covered on what SRM is a stout.
Why this surprises people building their first dark recipe
It's tempting to think "black beer" needs a grain bill dominated by dark malt, the way a pale beer is dominated by pale malt — but colour contribution and flavour contribution don't scale the same way. A grain bill that's 90%+ pale base malt by weight, with only a small measured percentage of roasted barley or another very dark grain, produces a genuinely black beer while keeping the flavour profile from becoming overwhelmingly roasty — which is exactly how most commercial stouts and porters are actually built. Overshooting on dark-grain weight in pursuit of colour is a common way to end up with an unpleasantly acrid, over-roasted beer that's darker than it needed to be for the colour target alone.
If your beer isn't reaching the colour you expected
Check the actual weight and Lovibond rating of whatever dark grain you used against the malt reference — a substituted grain, an inaccurate scale, or a different Lovibond rating than assumed accounts for most cases where a computed grain bill and the finished beer's actual colour don't match. See why is my beer darker than expected if the mismatch runs the other direction instead.