Most DIY overruns start with a tape measure and a guess. Roof pitch, stair geometry, and fence post counts are all closed-form geometry you can check before you order lumber.
Roof pitch is rise over run, usually stated as inches of rise per 12 inches of run. Rafter length is the hypotenuse of half-span and rise: thetoolss.com/tool/roof-pitch-rise-run-calculator
Stairs need even risers. Divide total floor-to-floor rise by a target 7–7.75 inch riser, then check the 17.5-inch rise+run rule of thumb and the stringer hypotenuse: thetoolss.com/tool/stair-rise-run-calculator
Decks are a coverage problem: board width including gap, joist spacing (16 inches on center is common), and two screws per joist intersection: thetoolss.com/tool/deck-board-joist-calculator
Fence posts are length divided by bay spacing, plus one. Rails scale with bays; pickets scale with pitch: thetoolss.com/tool/fence-post-spacing-calculator
Once the envelope is closed, insulation R-value and window U-factor tell you how hard the HVAC system will work: thetoolss.com/tool/insulation-r-value-heat-loss-calculator thetoolss.com/tool/window-u-factor-energy-calculator thetoolss.com/tool/hvac-cfm-room-calculator
Run the numbers in the browser, then confirm local code for rise, guardrails, and frost-line post depth.