Is there any practical way for a drinker to measure the caffeine in their own coffee?

The variables, in order of how much they matter:

The mass of coffee used. This dominates everything. Caffeine per gram of dry coffee varies within a fairly narrow band, so how many grams went into the cup is most of the answer. Weigh your dose and you have the biggest term.

Extraction efficiency. Caffeine is highly soluble and comes out early, so most brewing methods extract most of the available caffeine. This means the differences between methods are smaller than folklore suggests.

Serving size, obviously, and this is where café drinks diverge wildly.

The consequence that surprises people: a large filter coffee usually contains substantially more caffeine than a single espresso. Espresso is more concentrated per millilitre and much smaller. A double shot is a fraction of the dry coffee that goes into a mug of filter.

So weigh your dose and count your shots, and you can compare your own drinks reasonably well without any instrument.

Two pieces of received wisdom worth correcting while we are here.

Dark roast does not have meaningfully less caffeine. Caffeine is quite stable at roasting temperatures. There is a small effect because beans lose mass as they roast, so darker beans have marginally more caffeine per gram and fewer beans per scoop — which means measuring by volume versus by weight changes the answer more than the roast level does. Weigh, and the question largely disappears.

Species matters more than anything about preparation. Robusta contains roughly twice the caffeine of arabica. A blend with a high robusta content — common in supermarket espresso blends and in a lot of instant coffee — is a genuinely different drink in this respect, and it is the one variable that can double your figure without changing anything you do.

Direct measurement is possible and it is not a consumer proposition. The instruments that do it properly — laboratory chromatography, or the dedicated bench analysers used by producers — cost thousands, and that is the honest answer to the equipment question.

There are consumer test strips marketed for this. Reviews from people who have compared them against known references are not encouraging; they tend to give an order-of-magnitude indication rather than a number you could do arithmetic with.

So the realistic route is estimation from the variables that actually drive it, which turns out to be more informative than people expect, because the main driver is not the one everyone assumes.

For the practical question you are really asking — how much am I having — the useful move is to track what you can control and treat café drinks as a wide range.

At home, weighing your dose gives you a consistent baseline. Out, assume a wide interval, because the same drink name varies enormously between shops in both dose and shot count.

And if the reason for asking is that caffeine is affecting you, that is a conversation worth having with a doctor rather than solving with an instrument — individual sensitivity varies far more than the drinks do.