Calculating Line Exposure When a Banker Fails in a Perm

Perms are sold on the promise of covering your uncertainty across several matches, but the actual arithmetic of how many lines survive, or fail, when one selection goes against you is rarely spelled out clearly. Working through a specific example with real combinatorics removes the guesswork.

Setting Up the Example

Consider a standard 10-from-12 perm: you select 12 matches you are reasonably confident about, and the perm automatically generates every possible line using 10 of those 12 selections, covering you if up to 2 of the 12 go wrong. The number of lines generated by this structure is a straightforward combinations calculation, C(12,10), the number of ways to choose 10 items from 12 without regard to order.

So a 10-from-12 perm generates exactly 66 individual lines, each one a distinct combination of 10 of your 12 selections. At an illustrative 10p per line, that perm costs GBP 6.60 to cover in full; at 20p per line, GBP 13.20. Those figures scale linearly with whatever unit stake you choose to play.

What Happens When One Selection Fails

Now suppose one of your 12 selections, treated by you as a near-certain banker, actually goes wrong. The question is: how many of the 66 lines are now doomed, because they happened to include that failed selection among their 10 matches, and how many survive because they happened to exclude it?

The Verified Breakdown

A line is doomed if it includes the failed match among its 10 selections. To count those, fix the failed match as always included, then choose the remaining 9 slots from the other 11 matches: that is C(11,9).

So a single failed banker, out of 12 selections in a 10-from-12 perm, kills 55 of the 66 lines outright, roughly 83% of the full coverage, leaving only 11 lines, the ones built entirely from your other 11 selections, still alive. Those surviving 11 lines still need every one of their 10 chosen matches to be correct, since a single failure anywhere within a line still loses that specific line under standard perm scoring rules.

Why This Matters for Perm Selection

The lesson is not that perms are a bad structure, it is that the marketing framing of “covered against 2 wrong results” can understate how brutal the exposure is to a single failure in a smaller, tighter banker. If your 12 selections genuinely included two matches you were uncertain about and ten you were confident in, the maths above describes exactly the coverage you are paying for: protection against those two specific uncertain results going wrong, not blanket protection against any one of the twelve failing with equal safety.

A common mistake is treating every selection in a perm as equally protected. In reality, the structural coverage is symmetric across all 12 selections mathematically, but the practical safety net only feels adequate if your actual confidence genuinely matches the perms designed tolerance for exactly 2 unexpected results among the 12.

What If Both Weak Selections Fail?

Suppose you genuinely expected 2 of your 12 selections to be the risky ones, and both of them fail, exactly the scenario the perm is designed to tolerate. How many lines survive now? A line survives only if it excludes both failed matches entirely, meaning it must be built from the remaining 10 matches alone, choosing all 10 of them: C(10,10).

This is a genuinely important result for anyone relying on a perm’s headline promise. Even in the exact scenario the perm is marketed to protect against, both of your two anticipated risky results failing, only a single line out of 66 actually survives to pay out. That single line is precisely the one built entirely from your 10 confident selections, with no room at all for a third unexpected failure anywhere among those ten.

Scaling the Same Method to Other Perm Sizes

The same C(n,k) approach applies whatever size perm you are considering. An 8-from-10 perm generates C(10,8), 45 lines; a 9-from-11 perm generates C(11,9), 55 lines; an 11-from-13 perm generates C(13,11), 78 lines. Before committing to any perm structure, running this single calculation tells you exactly how many lines, and therefore exactly how much total stake, you are committing to, which is worth doing before looking at odds or form at all, since an unaffordable perm is not a realistic selection regardless of how good the underlying picks are.

Cost Versus Protection, Weighed Honestly

At 66 lines, even a modest 5p unit stake brings the full perm to GBP 3.30, while 50p a line brings it to GBP 33.00, a meaningful outlay for coverage that, as shown above, still collapses to a single surviving line in the exact failure scenario it is designed around. This is not a reason to avoid perms altogether; concentrated coverage of genuine uncertainty across 2 selections while trusting 10 others can be a perfectly reasonable structure. It is a reason to be precise about what you are actually paying for, rather than assuming the perm provides broad, forgiving protection across all 12 selections equally.

Whatever perm structure you use, verify the line count and cost before submitting, using the same C(n,k) combinations approach shown here, and treat the exercise as a way of understanding your actual exposure rather than a guarantee of any particular return. Keep stakes within a budget you have set in advance, and remember that pools perms remain a game of chance for adults aged 18 and over, regardless of how carefully the coverage has been calculated.