First order approximation: R-1 (British units BTU/hr/ft2/°F) per sheet of glass.
So double pane is R2, triple pane R3.
Second order approximation depends on the frame material, and to some degree on coatings, and gas fills, and years since the gas fill; also depends on your heating system, exterior wind exposure. It gets complicated enough that I have never bothered beyond the first order approximation.
Note that financially, replacing existing non broken windows does not pay off in terms of saving on your heating bill.
Consider: Let's talk about a 1 square meter window -- 10 square feet. Suppose that the replacement window costs $100 and you install it yourself. Suppose that you found a wonder window that was rated as R-5, and that your old windows are R-2.
If you are in my climate you have a 10,000 degree day heating season. That's 240,000 degree hours. The old window: 10 square feet * 240000 degree hours/2 = 1.2 million BTU per year. That's about 1/8 gigajoule or about 1.25 therms. Last year at peak winter rates, my gas bill was running $6/Gigajoule. So the old window cost me 75c per year.
If the new window was PERFECT, it would save me 75 cents per year. So the minimum payback is 133 years at zero percent interest. If I have to borrow money at even 1% the new window will never pay for itself.
Flaws in this reasoning:
If the old window is also a serious air leak, then it has a much smaller effective R value, and a correspondingly higher cost.
Windows such as aluminum sliders have a lower R value, often being effectively R-1. This would double the annual cost. Still not enough to warrant replacement.
While replacing the windows may not be cost effective they will make the house more comfortable. They will also make it easier to sell.
Window replacement is one of the LAST steps in making a house more energy efficient.
A much better strategy is to put shrink film plastic on the inside of your windows. This is only a few bucks per window, adds an R to the window, and also stops air leaks around the edges of the any windows that open. For a bit more you can get systems where you make frames of thin wood with magnets that stick to washers screwed to the existing frames. These are easy to remove for summer use. With more clever you make the add on frames 'double pane' with plastic on both faces, turning your R-2 window into an R4 window.
In winter when it is very cold out, the radiation through the window is significant. Having double layer window coverings can make a significant difference. In addition having a surface window treatment that reduces air circulation near the window allows the creation of a thicker boundary layer near the window, which will cool off, thus reducing the effective temperature differential between inside and outside.
Currently in the UK the electricity runs at 0.20£/kW-hr. My flat has 5 m2 of 30 years old double glazed windows, assuming a u-value of u = 3 W/m2/K, a winter outside T out of 4 °C and an inside T of 20 °C it gives 3(u) × 5(m2) × (20°C-4°C) × 24h × 0.20(£/kW-hr)/1000(W to kW conversion) = 1.15£/day. Make that for 180 days extended winter of the UK and you have 200£ spent each year. If I were to install new windows that would cost about 2500£ labour included and it would result in a saving of roughly a 100£ per year, as well as improve condensation, etc.