Levels and Squares

Most builders own one level and one square, then wonder why doors stick and joints won’t close. The skill isn’t owning a tool — it’s matching the right type and length to the job. Levels check for level (horizontal) and plumb (vertical), and tradespeople will often use a combination of various levels and squares to complete a project. A torpedo gets into tight spots. A box-beam runs framing. A speed square fences a crosscut. And every one of them needs an accuracy check before you trust it.

This guide covers the level and square types worth knowing, how to choose between them, and how to use them so the reading means something.

Levels: Pick the Type, Then the Length

The level you reach for depends on the job in front of you. Here’s how the main types break down.

Box-beam levels are the workhorses. The most common and versatile style is the box-beam level, constructed of a rectangular or box-shaped frame, usually extruded aluminum. They usually have a milled measuring edge that touches the surface, and many can be used vertically to measure plumb — a versatile tool for measuring a flat surface, whether vertical or horizontal. This is the level that handles framing and large surfaces.

I-beam levels are the budget-conscious general-use option. An I-beam level is best for general or light use; if you’re careful with them they can last a long time, but they need to be checked for accuracy more often than a box-beam level. If you need a level that won’t be used in a harsh environment or you’re price conscious, this is a great choice.

Torpedo levels earn their keep in tight quarters. When you’re working in tight quarters but still need a level measurement, you need a torpedo level — a 6- to 12-inch long level made just for working in small spaces. Most are rectangular with tapered edges, though some have a humped top that lets you use it in a smaller space or at an angle.

Screed levels are concrete tools. If you’re putting in a new concrete slab or basement floor, a screed level is the tool for you — it helps ensure your concrete is smooth and level, and can also be used for leveling sand and paving stones. Contractors regularly working on driveways, patios, and walkways should keep a screed level handy. That matters in Montana, where slabs poured in the short building season need to shed snowmelt and resist freeze-thaw.

String levels cover the long runs. When you need to find level across a very long distance, a string level is the tool to choose — meant to be hung or balanced on a piece of string stretched between two stakes or posts, pulled taut, so the level can check the long-distance level between the two posts rather than taking multiple measurements with a more traditional level.

Digital and magnetic options handle precision and metal. Digital box levels have the same case and construction as a box level, but in addition to a vial they give a digital readout to the fourth decimal place — the tool you need if you require a very precise measurement or have difficulty reading the bubble vial. A magnetic box-beam level holds itself securely to a magnetic surface, giving a fast, hands-free read on whether the object is plumb or level. A mason or wooden framing carpenter will have little use for magnets, but a commercial carpenter working steel framing will likely carry a trove of magnetic levels.

The practical setup? Builders and contractors should invest in a few quality levels — box or I-beam levels are the most versatile, but keeping a short or specialty level on hand can be a good idea depending on the types of jobs you do.

Squares: The Layout Tools That Decide Whether Joints Close

Squares are used for layout work to mark square (90°/right angle) and other angles, and to check for squareness and other angles during assembly. Different scales of work call for different squares.

Speed square — the one to buy first. Also known as a rafter square or triangle square, its triangular shape features a lipped fence that allows it to sit flush against a board’s edge, and it’s the most common type used across all manner of construction projects. The flange butts against a workpiece edge so you can draw a square cut line or 45-degree miter and use the square as a fence for crosscutting, while the diagonal edge has markings for laying out rafter, roof trim, and stair angles. The pro approach to sizing: use a 12-inch as a crosscut fence for saw and router, and keep a 6-inch in the apron pocket — especially handy up on a ladder.

Combination square — depth, center, and angle in one. A combination square is primarily used for ensuring the integrity of a 90° angle, measuring a 45° angle, finding the center of a circular object, finding depth, and simple distance measurements, and can also determine level and plumb using its spirit level vial. The head, with fences at 90 and 45 degrees, slides along the blade and locks, allowing you to transfer a distance accurately and hold it while you draw a line. For most general carpentry and DIY work, a speed square and combination square are often sufficient.

Framing square — the big-layout L. A framing square has an L-shaped design ideal for larger projects, consisting of a long blade and shorter, narrower tongue — handy for marking wide boards and sheet stock or checking squareness, and good for laying out rafters and stair stringers. It also handles determining roof pitch.

Drywall square — for sheet goods. With its 4-foot blade, a drywall square is ideal for laying out cut lines on standard-size sheets of plywood, drywall, and other board materials, and works as a guide when cutting drywall.

Try and engineer’s squares — fixed-90° precision. The try square is a fixed 90-degree square often used in woodworking for checking the squareness of edges and corners and marking perpendicular lines. An engineer’s square is a fixed 90° tool, typically hardened steel with a ground blade and stock, designed for ultra-precise work, often with tolerances measured in microns. For transferring any angle rather than a fixed one, sliding bevels let you replicate and transfer any angle, with the adjustable blade locking in place — perfect for bespoke joinery and trim work.

Square	Blade/Size	Primary Job
Speed square	6” apron / 12” bench	90° and 45° marks, crosscut saw fence, rafter layout
Combination square	Sliding head on rule	Depth, center of round stock, 45°/90°, built-in level vial
Framing square	L-shaped, long blade + tongue	Stairs, rafters, roof pitch, sheet goods
Drywall square	4-foot blade	Cut lines and cutting guide on drywall/plywood
Try / engineer’s square	Fixed 90°	Precision squareness checks, calibration

Choosing and Calibrating: What Separates a Good Tool From a Bad One

Two levels can look identical and read completely differently. Know what to inspect.

On levels, material and length come first. A good-quality spirit level can last many years if looked after — aluminum or box-section frames last longer than plastic ones, and if the vial becomes cloudy, leaks, or gives inconsistent readings, replace the level. That durability edge matters on cold-weather Montana jobsites where tools take knocks. For sizing, use the longest level that reasonably fits the job — bricklayers in particular benefit from longer, rigid levels to keep courses straight. A sensible starting point: start with a 600 mm (2 ft) level and add longer levels as needed, choosing a rigid aluminum or box-section frame, clear and readable vials, and a stated accuracy tolerance.

On squares, weight and rule straightness tell the story. Choose squares made from durable materials like aluminum or stainless steel for longevity; plastic squares can be accurate and lightweight but may not withstand heavy use or extreme conditions. Before you commit to a combination square, pick one up — the heavier the better. Plastic or cast zinc heads may lose their form the first time they’re dropped or knocked; a head of forged or cast steel provides longer-lasting straight faces. Then check the rule: hold the rule up to the light and sight along its edge, rejecting any with bows and bends, then grab the square by the head, tighten the thumbscrew, and try to move the rule — any slip or wiggle betrays an inaccurate tool.

Calibrating a level is a 30-second job that saves rework. Many assume a level is accurate just because it looks fine, but levels are often dropped or knocked out of calibration. Check by placing it on a flat surface, marking the bubble position, turning it 180 degrees, and checking again — if it reads differently, don’t use it for setting out.

Calibrating a combination square works the same way with a pencil. The first step is to check the square for accuracy: tape a white piece of paper to a board with a perfectly straight edge, place the anvil against the edge and draw a line on the outside of the blade, then turn the square over and draw a second line at least 1/32 inch away from the first. If the two lines are parallel, the square is accurate.

WBC carries a range of these layout pieces — including a 12-inch aluminum pro rafter square, an Empire 10-inch heavy-duty T-bevel, brass stair gauges, aluminum straight-edge rules in 48 and 72 inches, plumb bobs, and a steel square-head protractor.

Using Leveling Tools So the Reading Means Something

Even a calibrated tool lies if you use it wrong. The fundamentals:

Place the level firmly against the work, make sure there’s no mortar, dust, or timber underneath, let the bubble settle fully, and read at eye height — not from an angle. Reading from the side is a classic error. Looking at the vial from the side or above can make the bubble appear centered when it isn’t, so always read straight on at eye level — take a second if needed, because rushing leads to misalignment.

Don’t fight the tool. Pushing down on the level or bending it to match the work gives a false reading. Let the level sit naturally against the surface, and adjust the work to the level — not the other way around.

And don’t check once. In brickwork, check every course; in carpentry, check both directions when fixing frames or plates. Small errors compound quickly. That compounding is what shows up later as drift — the kind that becomes visible under heavy snow loads and seasonal movement. Take the time to get it right early; fixing mistakes later costs more time and materials than doing it properly the first time.

Frequently Asked Questions

What’s the first square a builder should buy? A 12-inch speed square. It’s versatile and unbreakable — it gives you 45- and 90-degree angles, it’s also a ruler, and it’s not hard to measure other angles with it. Many carpenters keep a 6-inch in the apron for ladder work and a 12-inch on the bench for larger layouts.

What level length should a new toolkit start with? Start with a 600 mm (2 ft) level and add longer lengths as needed. Go with a rigid aluminum or box-section frame and clear, readable vials, and confirm the tool lists a stated accuracy tolerance.

How do you know if a level is still accurate? Place it on a flat surface, mark the bubble position, turn it 180 degrees, and check again. If it reads differently, don’t use it for setting out. If the vial becomes cloudy, leaks, or gives inconsistent readings, replace it.

Aluminum or plastic — does it matter? For levels, yes. Aluminum or box-section frames last longer than plastic ones. For squares, durable materials like aluminum or stainless steel last longer, while plastic can be accurate and lightweight but may not withstand heavy use or extreme conditions. On a square’s head specifically, forged or cast steel provides longer-lasting straight faces than plastic or cast zinc, which may lose their form the first time they’re dropped.

Which level handles a concrete slab pour? A screed level. It helps ensure concrete is smooth and level, can also level sand and paving stones, and contractors regularly working on driveways, patios, and walkways should keep one handy.

Can one tool do both squaring and leveling? A combination square comes close. It ensures a 90° angle, measures 45°, finds the center of round stock, checks depth, and can determine level and plumb using its spirit level vial. For larger spans, a combination square-and-level tool with the square sitting in the middle gives a true 4-ft double-sided fence, letting you start layout at the very corner of your lumber and span wane and other irregularities.