Use our science-backed calculator before you squeeze anything (more details and caveats below it). You can calculate by weight or width (the shortest width of the whole fruit not the length between the pointy ends) – entering a 2-inch width lemon calculates a juice yield of about 45.97 grams, 1.62 ounces, 9.19 teaspoons, 3.06 tablespoons, 45.97 milliliters or 0.19 cups. If you’re using half a fruit remember to halve all the results displayed, and if you’ve cut it lengthways measure the shorter part of the flat side.
Sources, caveats and other info:
In the 1958 USDA Marketing Research Report No. 292, thousands of Lisbon, Eureka and Villafranca lemons (the primary cultivars in the US and virtually indistinguishable to each other in shape) that were commercially treated (washed & waxed) and stored in a commercial facility were analyzed in a variety of ways, one of which was juice content.
Lisbon, Eureka and Villafranca were initially 40.5, 45.2 and 44.8 percent juice by volume immediately after harvest, and quickly reached and stayed at around 50% over 1-6 months of storage (with Lisbon taking longer and gradually reaching 48.6% at the 6 month point). Given that produce is often stored for a long time (even as long as a year) before actually being sold, we used 50% for the calculator.
Now this is 50% by volume, but for practical purposes can be considered 50% by weight. When converting weight to volume outputs in the calculator, again for practical purposes density was assumed to be the same as water since the difference is small. And in case you're curious: if you include pulp - as would happen if you pulverize a whole peeled fruit - you would get around 3.7% more juice by weight (based on comparing USDA water content numbers between whole peeled lemons and just juice).
A caveat is that 50% is in a best-case scenario as the report above involved using straining machinery - in 2015 Lifehacker did an experiment that showed that squeezing by hand yielded almost 36% less juice than a handheld citrus presser.
For length-based inputs we turned to the 2014 paper Modeling physical properties of lemon fruits for separation and classification by Motie et al. in the International Food Research Journal which used regression analysis on the physical attributes of 70 samples each of Lisbon and Eureka Lemons to come up with an equation for the relationship between their minor diameter (the shortest width) in millimeters and mass in grams. This could then feed into the 50% figure above.
However these calculations may not work for some other less-common varieties (even though they may be common outside the US) because they have different shapes, juice percentages and width-to-mass ratios - for example in the book Citrus Fruit: Biology, Technology and Evaluation by Milind Ladaniya published by Elsevier's Academic Press in 2007, Indian "Galgal" varieties only have 24-25% juice.
And one teaspoon may not be the same as another teaspoon either - acidity can also vary, not just between varieties but also within the same variety because of storage conditions. According to the 1961 paper Effect of Temperature and Holding Period on Some Physical and Chemical Characteristics of Lemon Fruits by Irving L. Eaks published in the Journal of Food Science, lemons stored in warmer temperatures (24 degrees Celsius or 75.2 degrees Fahrenheit) became more acidic to a greater extent than those stored in cooler temperatures (4 and 13 degrees Celsius or 39.2 and 55.4 degrees Fahrenheit) over 4, 8 and 12 weeks. Lemons stored in cooler temperatures that were then transferred to warmer temperatures after the 12 weeks also accelerated their increase in acidity. Now we aren't recommending that you store them for weeks on end, but in general what this study seems to suggest is that (for a few days perhaps) you should store them at around room temperature for more acidic lemons, and in the fridge if you want the opposite.