Worker-turns

=== Definition ===
The length of time it takes Workers to finish a tile improvement such as Roads, Irrigation, etc., is called a worker-turn. For example, if it takes 3 turns to finish Irrigation, then it can be said that Irrigation takes 3 worker-turns to finish.
=== Advanced Explanations and Mechanics ===
==== Units and measures ====
When measuring the time to complete a tile improvement, the worker-turn measurement assumes the Workers unit without any extra bonuses from veteran levels or work-rate bonuses. Unit bonuses such as the 2x work-rate bonus of Engineers are not included in worker-turn measurements for tile improvements. Instead, Engineers are said to have a work-rate of 2 worker-turns per turn. (A tile improvement which takes Workers 10 turns would finish in 5 turns if done by Engineers, but the tile improvement is still said to take 10 worker-turns.)
==== Source of the work-rate of units ====
The work-rate of a unit in worker-turns starts directly from its base movement rate, ignoring all movement bonuses it may acquire. After acquiring this base work-rate, bonuses from veteran levels and other work-rate bonus effects are multiplied over it, resulting in the final work-rate of a particular unit.
==== Rounding ====
The time it takes to finish a tile improvement will be the rounded up value from:
(Time to build tile improvement in worker-turns) ÷ (work-rate of the unit in worker-turns per turn)
For example, for an Engineer with a work-rate of 2 worker-turns per turn to finish a Mine whose time is 5 worker-turns:
5÷2 = 2½, which rounds up to 3 worker-turns. Thus, in this example it would take Engineers 3 turns to finish the Mine.
==== Combined Work ====
Multiple units can combine their work-rate into producing a tile improvement. Units whose work is interrupted before the tile improvement is completed, do not donate any worker-turns to its completion. However, units who arrive late and start donating their work-rate on the turn they begin their work, start adding that work-rate to the completion of the task. For example:
If Irrigation takes 3 worker-turns to finish and one Workers unit starts it on Turn 1, there will be 2 worker-turns left on Turn 2. An additional Workers unit can join the irrigating activity at Turn 2 and together the two Workers units will put in 1+1=2 worker-turns per turn, allowing them to finish the Irrigation on Turn 3 instead of Turn 4.