Software Estimation: Demystifying the Black Art (Best Practices (Microsoft))

Use lines of code to estimate size, but remember both the general limitations of simple measures and the specific hazards of the LOC measure in.

Count function points to obtain an accurate early-in-the-project size estimate.

Use the Dutch Method of counting function points to attain a low-cost ballpark estimate early in the project.

Use GUI elements to obtain a low-effort ballpark estimate in the wide part of the Cone of Uncertainty.

With better estimation methods, the size estimate becomes the foundation of all other estimates. The size of the system you're building is the single largest cost driver. Use multiple size-estimation techniques to make your size estimate accurate.