How do I calculate absolute risk and NNT for my own situation?

The short version: you can calculate your own absolute risk and NNT, but the hard part is finding an honest baseline risk for your specific situation, not the arithmetic.

The math is simple. Absolute risk is your chance of the outcome without treatment, minus your chance with treatment. NNT is 100 divided by that difference, when both are expressed as percentages. If your baseline risk is 20 in 100 and treatment drops it to 10 in 100, the absolute reduction is 10 in 100 and the NNT is 10. If your baseline risk is 2 in 100 and treatment drops it to 1 in 100, the reduction is 1 in 100 and the NNT is 100 [3].

The trap is the baseline. Most published NNTs come from trials on specific populations, and the same relative reduction maps to wildly different NNTs depending on who is treated [3]. A drug that looks impressive in a high-risk trial can be nearly worthless in a low-risk patient. So you need three honest inputs: your baseline risk over the relevant time window, the relative risk reduction from the intervention, and the harm rate. The first is usually the weakest link. Your doctor may quote a risk calculator, but those models are only as good as their calibration, and a miscalibrated model can produce net harm when used to decide treatment [13].

The evidence retrieved here is thin for your exact question. It gives worked NNT examples for pain drugs, bipolar depression, and Alzheimer's [4, 5, 6, 8], but no tool for personalizing your own baseline. What matters is that you demand the absolute numbers, not just the relative reduction, and that you ask who funded the trial that produced the NNT. Industry-funded studies are marketing with a Methods section.

My call: do the arithmetic yourself, but treat any baseline risk you are given as an estimate with real uncertainty, and ask for the absolute numbers behind every NNT. Confidence: high on the method, low on the availability of honest personalized baselines.