Project Guides
Long-form guides that explain the measuring, sizing, and buying decisions behind the calculators — written from the same published sources the tools are built on.
The calculators on this site compute; these guides explain. Each one walks through the reasoning behind a common estimating or sizing decision — where the published numbers come from, which conventions are safe to lean on, and where the shortcuts break down — so you understand the answer a calculator gives you, not just receive it.
Every factual claim in a guide traces to the same published manufacturer data and standards the corresponding calculators are verified against, and each guide links the tools that do the arithmetic.
All Guides
Which Stick Electrode Should You Use? Reading AWS Welding Rod Numbers
How to read an AWS stick electrode number — tensile strength, position, coating, polarity — and why the amperage window comes from the manufacturer.
Updated August 22, 2026
How Many Wires Fit in a Conduit? NEC Fill Percentages Explained
Where the 40% conduit fill limit comes from, why one wire gets 53%, and how NEC Chapter 9 turns wire areas into a maximum conductor count.
Updated August 22, 2026
What’s the Difference Between Grounding and Bonding? Two Wires, Two NEC Tables
Why a panel has two different ground wires: the grounding electrode conductor from NEC Table 250.66 and the equipment ground from Table 250.122.
Updated August 22, 2026
How Does an O-Ring Seal? Dash Numbers, Squeeze, and Groove Fill
How an O-ring actually seals and what AS568 dash numbers encode — squeeze, gland fill, and stretch, and where groove dimensions come from.
Updated August 22, 2026
How Does a Press Fit Hold? Interference, Friction, and Shrink-Fit Math
How an interference fit grips: ISO 286 fit classes, Lamé interface pressure, the torque it can carry, and the shrink-fit heating temperature.
Updated August 22, 2026
How Do You Read a Pump Curve? System Curves, Duty Points, and NPSH
How a pump curve and a system curve meet at the duty point, and why the NPSH-required line decides whether your pump cavitates.
Updated August 22, 2026
How Do You Read a Span Table? Loads, Spacing, and Deflection Limits Decoded
What the rows and columns of an IRC span table mean — loads, spacing, deflection limits, and how to check a joist, rafter, or header against one.
Updated August 22, 2026
How Do You Calculate Speeds and Feeds? RPM, Chip Load, and Feed Rate
The two equations behind every speeds-and-feeds chart — spindle RPM from surface speed and diameter, feed rate from chip load and flutes.
Updated August 22, 2026
Why Do Bolt Torque Specs Matter? Preload, Friction, and the K-Factor Explained
Why torque specs exist: the T = K·F·d short-form equation, preload as a fraction of proof load, and how lubrication changes the number on the wrench.
Updated August 20, 2026
Which Pipe Material Should You Use? PVC, Copper, Steel & PEX Through the Numbers
PVC, copper, steel, and PEX compared the way the equations see them — friction C-factors, inside diameters, thermal movement, and surge stiffness.
Updated August 20, 2026
Conduit Bending Math — Offset Multipliers, Shrink, and Stub-Up Take-Up
Where the 30-degree offset multiplier comes from, what shrink does to your measurements, and how take-up deducts place a 90-degree stub mark.
Updated August 20, 2026
How Strong Is a Fillet Weld? Throat Size, Weld Lines, and Eccentric Loads
What makes a fillet weld strong — throat vs leg size, treating welds as lines, and why eccentric loads multiply the stress at the far corner.
Updated August 20, 2026
What Does No. 2 Lumber Mean? Grades, Species Groups, and Span-Table Strength
What No. 2 grade means in span tables, why the IRC treats four species groups as one, and how dressed sizes shrink from their nominal names.
Updated August 20, 2026
Why Do Sheet Metal Gauge Numbers Run Backwards? Gauge Systems Explained
Why gauge numbers shrink as steel gets thicker, why aluminum uses a different system, and what thickness really feeds a bend-allowance layout.
Updated August 20, 2026
How Do You Pick a Tap Drill Size? The 75% Thread Engagement Rule
The percentage-of-full-thread formula behind every tap drill chart, why 75% engagement is the default, and when to drill oversize on purpose.
Updated August 20, 2026
How Do You Read the NEC Wire Ampacity Table? 310.16 Columns, Corrections & Limits
How to read NEC Table 310.16 — insulation temperature columns, ambient correction, bundling adjustment, and the termination limit that caps it all.
Updated August 20, 2026
How Do You Calculate Beam Deflection? Simply Supported & Cantilever Formulas
The four closed-form beam deflection formulas — 5wL⁴/384EI and its siblings — worked through a steel-beam example, with unit discipline and code limits.
Updated August 16, 2026
Concrete Mix Ratios Explained (ACI 211) — Water-Cement Ratio & SCMs
Why 1:2:3 isn’t a design: how ACI 211.1 proportions a mix from the water-cement ratio, slump, and aggregate size, with fly ash and slag replacement math.
Updated August 16, 2026
Rebar Development Length & Lap Splices (ACI 318-19)
How ACI 318-19 turns bond into a bar length: the general equation’s five ψ factors, the confinement cap, Class A vs B laps, and one 16 mm bar worked through.
Updated August 16, 2026
How Does Manning’s Equation Work? Open-Channel Flow Step by Step
Manning’s Q = (k/n)·A·R^(2/3)·√S taken apart term by term — worked on a concrete channel, with the Froude number and the strange case of the almost-full pipe.
Updated August 16, 2026
Ohm’s Law and Watt’s Law — The Complete Guide
V = IR and P = VI, and the twelve formulas they generate — worked through a 1,500 W heater, an overloaded extension cord, and the reason wires get hot.
Updated August 16, 2026
Punching Shear in Flat Slabs (ACI 318) — Two-Way Shear Explained
How a column tries to punch through a flat slab: the d/2 control perimeter, ACI 318’s three vc limits, and one interior-column check worked to a verdict.
Updated August 16, 2026
How to Read Steel Beam Sizes — W-Shapes, IPE & HEA
What W14x30, UB 356x171x51, and HEA 300 actually encode — nominal vs real depth, the five properties that matter, and one beam taken from label to capacity.
Updated August 16, 2026
U-Value vs R-Value — How Thermal Transmittance Is Calculated (ISO 6946)
R resists, U transmits, and one is the other upside down — the layer-by-layer ISO 6946 method, the imperial/metric R-value trap, and why studs cost a third.
Updated August 16, 2026
What Is the NEC Voltage Drop Rule? The 3%/5% Guideline Explained
Where the NEC’s 3% branch and 5% total voltage-drop figures come from, why they aren’t mandatory, and how a 100-ft circuit fails them — worked in AWG sizes.
Updated August 16, 2026
Wind Load Calculation Explained — ASCE 7-22 vs Eurocode 1
How ASCE 7-22’s K-factor chain and Eurocode 1’s peak velocity pressure both turn a wind speed into design pressures — worked side by side on one building.
Updated August 16, 2026
Asphalt vs. Gravel vs. Concrete: Three Ways to Buy a Driveway
One driveway question, three units of purchase — gravel by the yard, asphalt by the ton, concrete by the pour — and the depths and allowances behind each.
Updated August 11, 2026
Drywall Estimation Basics: Sheets, Mud, and the Butt-Joint Problem
How board area really adds up, why sheet length matters more than price, what 10% waste covers, and how framing spacing quietly sets your screw count.
Updated August 11, 2026
Fence Planning Basics: Posts, Rails, and Pickets
How a run length becomes posts, rails, and pickets, what the 1/3-height bury rule means for post length, and why concrete is bought once, on the total pour.
Updated August 11, 2026
How Much Extra Flooring Do I Need? Waste Factors by Pattern
Why plank and tile waste runs 10% straight, 15% diagonal, and 20% herringbone — and why broadloom carpet follows roll geometry instead of a flat percentage.
Updated August 11, 2026
Framing Lumber Math: Studs, Headers, and Board Feet
How a wall length becomes a stud count, why header depth comes from a code table while the takeoff counts stock, and how board feet price the lumber list.
Updated August 11, 2026
Gravel Base and Paver Patios: What Goes Under Matters Most
How base depth follows traffic and soil, why compacted gravel needs a 1.15 over-order, what the 1-inch bedding layer is for, and how lay patterns set waste.
Updated August 11, 2026
How Much Concrete Do I Need? Slabs, Footings, and the Bag Math
How slab, footing, and step volumes are figured, when bagged premix stops making sense, what the waste allowance covers, and where mix design enters.
Updated August 11, 2026
How Much Paint Do I Need? A Room-by-Room Walkthrough
How to measure walls, subtract doors and windows, pick a realistic spread rate, and decide on primer and coats — so you buy the right number of gallons.
Updated August 11, 2026
How Much Mulch Do I Need? Depth Decides Everything
Why spread depth drives every mulch and topsoil order, how bags compare with bulk delivery, what a yard really weighs, and how new lawns change the math.
Updated August 11, 2026
Sod vs. Seed: What Starting a Lawn Really Takes
What sod really costs in rolls and pallets, how seed rates swing by species, and how planting windows, slopes, and budget decide which route your lawn takes.
Updated August 11, 2026
Stair Layout Basics: Rise, Run, and the Code Chain
How a floor-to-floor rise becomes a riser count, why the IRC’s 7¾-inch limit governs the whole flight, and how the guard’s 4-inch rule finishes the stair.
Updated August 11, 2026
Topping Up Attic Insulation: Buy the Shortfall, Not the Target
How to read the insulation already in your attic, pick a target R-value, and turn the shortfall into bags using manufacturer coverage charts.
Updated August 9, 2026
Cord vs. Face Cord: Buying Firewood Without Getting Shorted
What a legal cord of firewood is, why a face cord’s value hinges on piece length, how to measure a delivered stack, and how to compare wood by heat, not volume.
Updated August 9, 2026
Deck Framing Spans 101: How the Load Gets to the Ground
The load path from decking to soil, what the IRC R507 span tables assume, how species and spacing change spans, and where prescriptive deck design ends.
Updated August 9, 2026
Why Your Boat Won’t Go Faster: A Diagnosis in Three Numbers
Why displacement hulls hit a wall at 1.34·√LWL, what Crouch’s formula says planing speed costs, and how prop slip diagnoses a slow boat.
Updated August 9, 2026
Pool Water Balance Basics: Gallons, ppm, and Four Levers
Why every pool dose starts with your gallons, what ppm really means, which chemistry levers you can raise by the pound, and why lowering pH is different.
Updated August 9, 2026
Why Your Room Sounds Bad: One Room, Two Problems
Room modes and reverberation time in plain terms — why small rooms have lumpy bass, what RT60 targets mean, and where the Schroeder frequency splits the job.
Updated August 9, 2026
Window AC Sizing: The ENERGY STAR Method, Explained
How the ENERGY STAR room-AC table works, what the sun, occupancy, and kitchen adjustments really adjust, and why an oversized unit leaves a room clammy.
Updated August 9, 2026