On the Digital SAT, every Math question allows a calculator. For most students that means Desmos inside Bluebook, not a TI-84 in their bag.
Desmos is worth learning. It's also easy to overuse: if you type every problem into the calculator, you'll run out of time on questions that were solvable in a few steps on paper.
Opening the calculator in Bluebook
The calculator appears only during the Math section. Bluebook has a Calculator button that opens Desmos in a panel on screen (exact layout can vary slightly by device, but the control is always on the Math interface).
Reading & Writing has no calculator.
The Question Menu lets you jump to any question in the current Math module and return to flagged items. During a Bluebook practice test, check what happens to your calculator entries when you switch questions. Write down coordinates you'll need again.
Bluebook runs offline during the test; the calculator does too.
If you've never clicked that button, do it during a Bluebook practice test, not on test morning.
What to use it for
Bluebook gives you a graphing calculator, not just arithmetic keys.
Systems of equations (Algebra). Enter something like `y = 2x + 3` and `y = -x + 9` on separate lines. The intersection point is often the `(x, y)` the question wants. Watch your signs when you type; a wrong sign still produces a clean-looking graph.
Quadratics and functions (Advanced Math). If the problem gives you `f(x) = x² - 4x + 3`, graph it. Questions about zeros, the vertex, or how many solutions the equation has are often easier to read off the graph than to factor under time pressure. To solve `x² - 4x + 3 = 0`, graph `y = x² - 4x + 3` and find the x-intercepts.
Constants and parameters (Advanced Math). If a question asks for the value of `k` such that the line `y = k` intersects the parabola `y = x² - 4x + 1` at exactly one point, graph both in Desmos. A single intersection means the line touches the vertex; read the y-coordinate there (`k = -3` in this example) instead of completing the square under time pressure.
Tables (Problem Solving & Data Analysis). For "what is `f(3)`?" or comparing a few inputs, Desmos's table view saves retyping the function. Plotting `(x, y)` pairs from a stem is useful when the question asks whether a linear model fits the data.
Coordinate geometry (Geometry & Trigonometry). Enter the circle and the line from the question—for example `(x - 1)² + (y + 2)² = 16` and a line through two given points—then read the intersection coordinates from the graph.
Window and zoom. If everything looks piled on the axis, zoom out or in before you decide there's no intersection.
When to skip it
Calculator allowed does not mean calculator required.
Simple linear equations (`3x = 15`), small-integer "which choice works" problems, and basic percent questions are often faster by hand. On geometry items with a diagram, mark the figure first; graph only if the relationships aren't obvious from the picture.
If you can see the path in a few steps, take it. Open Desmos when the algebra is messy, the question is built around a graph, or you want to check work you're unsure about.
For how to prioritize Math topics from a practice test, see How to Improve Your SAT Math Score.
Desmos or a handheld calculator?
Bluebook's Desmos is enough for the full Math section. College Board still publishes a list of approved external calculators if you want one.
Some students use Desmos for everything because graphing systems and quadratics is faster than a basic scientific calculator and there's nothing to pack or charge. Others use a handheld for quick arithmetic and Desmos for graphs. Either works if you've practiced that way before test day. What fails is bringing hardware you've never used and Desmos you've never opened.
Practice
Run at least one or two full Bluebook practice tests with the calculator open on purpose: resize the panel, graph a handful of Math questions, get used to switching between Desmos and the question.
When you do timed sets, pause before opening Desmos and ask whether graphing is worth the seconds.
For a short drill: take five Math questions you missed on a practice test. Before reading the solution, write yes or no on whether Desmos would have helped, and which feature (graph, table, intersection). Then check the official explanation and adjust your rule.
College Test Coach includes Desmos on Math practice and review screens, so you can run that drill on weak skill areas between full Bluebook tests.
Mistakes that cost points
Copy equations carefully from the stem; one sign flip yields a wrong answer that still looks reasonable.
Reread the last line of the question after Desmos gives you a number. Many items ask for `2x`, `f(4)`, or a sum, not `x` alone.
Close the panel if it obscures information you need in the question.
Desmos doesn't replace drawing on diagrams for word problems. Use paper markup and the graph when each helps.