Section 1
The calculator: five inputs, one fragility score
Do this on a single sheet. You need your payroll broken into pay bands. Most operators can pull this from payroll software in minutes; if not, list every employee with their annual pay and sort them. Input 1: Band your heads Sort every employee into four annual-pay bands. The band edges are chosen around the UK employer NI thresholds, because those are the lines policy actually moves. Adapt the edges to your own country's equivalent thresholds if you are not in the UK. Count the heads in each band. The counts are your raw material. Input 2: Compute current employer NI per band For each employee, employer NI is roughly 15 percent of pay above 5,000 pounds (the current rate and secondary threshold; Deloitte TaxScape on the Autumn Budget 2024 measure; Xero UK summary). Sum it by band. You now know how much NI each band contributes today. Input 3: Compute the "threshold-move" sensitivity Here is the move that reveals fragility. Ask: if the secondary threshold dropped again, say from 5,000 to 3,000, how much new NI appears? Every head in every band picks up 15 percent of 2,000, or 300 pounds, but the ones in Band A and low Band B are the ones switching on from near-zero. Run the same test for a threshold rise back toward 9,100 to see your upside. The size of the swing per 1,000 pounds of threshold movement, multiplied by your head count, is your threshold sensitivity. A payroll with many heads clustered near the threshold swings hard. A payroll of a few high earners barely moves. Input 4: Compute the "rate-move" sensitivity Now the other kind of change. If the rate rose from 15 to 16 percent, how much new NI appears? It is 1 percent of all pay above the threshold, across everyone. This scales with total pay, not head count, so it hits big and small payrolls in proportion. Note the figure. It is your rate sensitivity. Input 5: The fragility ratio Divide your threshold sensitivity by your rate sensitivity. • Ratio well above 1: your exposure is concentrated in threshold moves, which means it is concentrated in headcount shape. You are a shape-fragile payroll. A future threshold cut hurts you far more than the average business, and a rate cut helps you less. • Ratio around or below 1: your exposure is mostly to rate moves, which track pay. You are a size-exposed payroll. You feel changes roughly in line with your wage bill, no worse. That single ratio is the fragility figure. It tells you, in one number, whether the next budget's shape matters to you more than its size. Write it at the top of the sheet. It is the output of the whole exercise.
Section 2
The breakpoint: the number to watch
The fragility ratio tells you how sensitive you are. The breakpoint tells you where it starts to bite. Compute it in two steps. First, take your rebuilt contribution margin per unit on today's rules, revenue per unit minus the variable costs that move with the sale, including current employer NI per unit. (If you have not rebuilt this, that is the prerequisite, and the full reset playbook covers it.) Second, ask how much a threshold or rate move would add to your labour cost per unit, using the sensitivities you just computed, and find the move that drives your contribution margin to zero. That policy move, expressed as "a threshold cut to X" or "a rate rise to Y percent," is your breakpoint. It is the specific, named policy change that takes your unit economics underwater. Most operators have never expressed their risk this way, and it changes the conversation entirely. Instead of "another tax rise would be bad," you have "a threshold cut below 3,500 pounds, or a rate above 17 percent, breaks my model at current volume." That is a figure you can watch a budget against, plan against, and pre-decide against.
Section 3
Worked example: the April 2025 shock in the rear-view
Run the calculator backwards on a shock that already happened and you can check it works. Before April 2025 the secondary threshold was 9,100 pounds and the rate was 13.8 percent. The change dropped the threshold to 5,000 and raised the rate to 15 percent (Deloitte TaxScape; Xero UK). A restaurant with forty part-timers, many earning between 6,000 and 12,000 pounds, had most of its heads in what are now Bands B and C. Its threshold sensitivity was enormous, because a big share of its workforce sat right on the line the policy moved. Its fragility ratio would have read well above 1, flagging it as shape-fragile, before the budget landed. Take one worker to sanity-check the mechanics. On a 20,000 pound salary the old employer NI was 13.8 percent of 10,900, or 1,504 pounds. The new figure is 15 percent of 15,000, or 2,250 pounds, a 50 percent rise for the same job (The Access Group hospitality analysis). And a part-timer on 6,000 pounds went from zero employer NI to 150 pounds, because the threshold dropped below their pay. A calculator built the way this one is would have shown that restaurant, in advance, that a threshold move was the change it could not afford, and that its fragility lived in the forty heads, not in the wage total. The operator who had run the number would have been reshaping the rota in the autumn, not reacting in the spring. One relief to fold in before you act on the figure. The Employment Allowance rose to 10,500 pounds in April 2025 and the old 100,000 pound eligibility cap was removed, so many smaller employers offset a large slice of employer NI and some pay none (The Access Group). Subtract your allowance before you read your breakpoint, because for the smallest operators it moves the breakpoint a long way out.
Section 4
The two disciplines under the tool
The calculator is comparative statics wearing work boots: change one policy variable, hold everything else still, and read exactly where the cost lands. Its strength is that it forces you off "taxes went up" and onto the specific lever, threshold or rate, and the specific part of your payroll it moves. Its limit is that the world does not hold still. Staff, customers and competitors all react, so the breakpoint is your opening exposure, not a forecast of where you end up. Re-run it as things move. The fragility ratio is a crude sensitivity analysis, the same logic a lender runs on a loan book. It cannot tell you what policy will actually do. It can only tell you how much a given move would hurt. That is enough to decide where to spend your defensive effort, which is the only decision the tool is for.
Section 5
What the calculator cannot see
It cannot see the next policy. It prices your exposure to a change in the rules, not the odds of that change happening, because those odds come from a political system that does not read your payroll. Treat the output as a robustness measure, not a prediction. It also assumes your pay bands are stable; a business mid-restructure has a moving denominator, so re-band before you trust the ratio. And it says nothing about whether you should absorb, pass through, or reshape in response, those are the operating decisions the fragility figure informs but does not make.
Section 6
The fitness test
You have run the number properly if you can state your fragility ratio in one figure and name your breakpoint as a specific policy move, "a threshold cut to X breaks my margin," backed by your own banded headcount. If you can, you will read the next budget as an operator with a threshold to watch, not a citizen bracing for bad news. You have not run it if your sense of payroll risk is still "the wage bill is high." That is a feeling, not a figure, and it points at the wrong lever, cutting pay, when your real exposure may be the shape of the rota. The operators who navigate the next round of policy volatility will be the ones who turned their payroll into a fragility number and a breakpoint before the change arrived, and pulled the matching lever early.