The calculator that thinks in lines
Most workdays in engineering, science, finance, and any technical trade have the same thread running through them: a constant drip of small calculations. Converting between units. Checking that a back-of-envelope estimate still holds up. Computing the geometric mean of a few numbers. Differentiating a function to find where it's flat. Asking what 60 mph actually is in metres per second, again, because nobody really remembers.
The tools we reach for to do this work are not very good at it. The Windows calculator app gives you one answer at a time and forgets it the moment you press equals. Excel demands cells and rows and column headers for what should be a single line. A Python REPL gives you everything except a way to see what you typed three minutes ago. The web calculators are full of advertising banners and the desktop scientific calculators look like they were last updated in 2003.
Zenith Reckon is built to be the tool you actually reach for in that situation. It is a scratchpad calculator: one window, two columns, expressions on the left, answers on the right, and your previous work always in view. It treats math the way you actually do math when you're thinking through a problem on a napkin—line by line, building on what came before, without ceremony.
Think in lines.
A scratchpad, not a calculator
The interface is deliberately quiet. There are no buttons for digits, no calculator-pad layout, no settings dialog. You type expressions; results stream in on the right as you type. Each line is one calculation. Names you define on one line are usable on every line after it. Comments after a # are ignored, so you can leave breadcrumbs for yourself:
# Quick sizing for the cooling loop
flow = 18 L/min
density = 998 kg/m^3
mass_flow = flow * density # kg/min
mass_flow to kg/s # convert when you want
That last line is the move that defines the whole product. You don't multiply by 60 in your head. You don't open a converter. You write "to kg/s" at the end of the expression and Reckon does the unit conversion for you, in the same line, as you type. Then you move on.
Units that just work
Engineering math is unit math. The thing that separates a useful technical calculator from a toy is whether it understands that 5 mph + 3 m/s is a meaningful expression, not an error. Reckon does. The unit system is built on Pint—the same library that physicists and chemists use in their Jupyter notebooks—so the unit catalogue is comprehensive and the dimensional checking is rigorous. You can mix imperial and metric, derived and base, scientific and everyday, and the result comes out in consistent units.
acceleration = 9.81 m/s^2
distance = 1/2 * acceleration * (3 s)^2
distance to ft # comes out in feet
If you add quantities with incompatible dimensions—mass to length, say—Reckon flags it on that line as an error, in red, without interrupting the rest of your sheet. Your work above stays evaluated; only the broken line shows the problem. Fix the typo and the red goes away.
Variables, functions, and a small numerical lab
Variables persist down the page. You name something on line 3 and use it on line 47; if you change the definition on line 3, every line below it recomputes instantly. The dependency graph is just top to bottom—simple to predict, easy to debug.
User-defined functions follow the same syntax that mathematicians have used on whiteboards for a century:
f(x) = x^3 - 4x + 1
f(2) # plug in a value
solve(f, 1) # find a root near x = 1
diff(f, 2) # numerical derivative at x = 2
integral(f, 0, 3) # integrate from 0 to 3
The numerical methods underneath are the standard ones—Newton's method for root finding, central differences for derivatives, Simpson's rule for integration—at fixed sensible tolerances. Reckon is not pretending to be Mathematica. The point is that you can sketch a function and immediately ask the obvious quantitative questions about it, in the same window, without leaving the scratchpad.
Statistics arrive in the same understated way. mean([12.3, 11.7, 12.1, 12.4]) returns the mean. stdev, median, variance, and mode are there too, doing what they say. Vector operations—norm, dot, cross—handle the geometry you do when you have a unit vector or a force in three dimensions and need a magnitude or a projection. Complex numbers use the natural 2 + 3i literal syntax that engineers write in textbooks, not Python's 2 + 3j.
Physical constants on tap
Fifteen CODATA-recommended physical constants are built in by name: c for the speed of light, h for Planck's constant, k_B for the Boltzmann constant, N_A for Avogadro's number, G for the gravitational constant, g_n for standard gravity, and so on. They carry their units with them, so they compose with the rest of your expression without conversion.
# Photon energy at 532 nm
wavelength = 532 nm
energy = h * c / wavelength
energy to eV # green laser photon, ~2.33 eV
Constants can be locally shadowed by your own variables—a deliberate choice, since sometimes h is a height and not Planck's constant—but if you don't redefine them they're always there.
A tool that lives at the side of your screen
Reckon was designed to dock against the edge of a monitor and stay there. The window remembers your work between sessions, so the next time you open it your previous calculations are still in front of you. The header collapses cleanly at narrow widths so the docked column doesn't waste pixels. The body is keyboard-first: Ctrl+E exports the current scratchpad to a PDF, Ctrl+L clears it, F1 opens the in-app manual, and that is essentially the entire shortcut set you need to know.
The PDF export is the working half of what would otherwise be ephemeral. Every scratchpad can be saved as a clean two-column document—expression on the left, result on the right, dated, with a small footer—suitable for attaching to a report, a notebook entry, or an email. The format is intentionally plain so the math is what stands out, not the styling.
What Reckon deliberately doesn't do
Reckon is not a computer algebra system. It does not factor polynomials symbolically, simplify expressions, take symbolic derivatives, or solve equations in closed form. If the work requires manipulating expressions as symbols, the right tools are SymPy, Mathematica, or Maple. Reckon's calculus is strictly numerical.
Reckon is not a plotting tool. It computes scalar quantities, not curves and surfaces. There is no chart panel, no axes, no fitted lines on a graph. The right tools for that work are matplotlib, Plotly, or Excel's chart features.
Reckon is not a notebook. There are no rich text blocks, no embedded images, no inline equation rendering, no narrative scaffolding around the math. A scratchpad is a different artifact from a notebook; trying to be both would make either worse. If you need a record of why you did the calculation, write it next to the expression as a comment, or paste the calculation into a real document.
Reckon does not collect telemetry. The application is a single executable that runs locally, reads and writes one file in your home directory for persistence, and never reaches outside your machine. Whatever you compute on it stays on your machine.
The design philosophy
The premise behind Reckon is that the tiny calculations are not actually tiny. They are most of the cognitive work in technical thinking. Engineers and scientists run more arithmetic in a single morning than they ever realise; the cost of running it through a tool that erases its own history is that the work has no surface, no continuity, no trail. Each calculation is a fresh start.
A scratchpad that keeps the line above visible while you write the line below is a different thing. It lets you build, refine, check, revise. It lets you write the conversion factor in once, name it, and use it everywhere. It lets you spot the calculation you got wrong, because the calculation that depends on it is right there, contradicting itself. Treating a calculation as a small editable document instead of a transient keypress is, in the end, the whole product.
That is the kind of tool worth having open on your second monitor every day.
Zenith Reckon is a desktop scratchpad calculator for engineers, scientists, and students who do real math throughout the day, available for Windows on perpetual licensing.