Interactive tool
Boundary & Equivalence Generator.
You cannot test every value, so you test the right few. Describe an input rule and this gives you the exact set that covers it: one value per equivalence partition, every boundary and one step past it, and the invalid cases people forget. Each value comes with the reason it earns its place. Copy the set straight into your cases. Your work saves in your browser, and nothing you type leaves it.
Describe the input
Field guide
Cover the space with the right few values
This is the technique interviews probe and the one that separates a thorough tester from a hopeful one. The generator does the arithmetic; the thinking is yours.
Equivalence partitioning
Group the inputs the system should treat the same way, then test one value from each group. If 6 behaves like 7 behaves like 14, you do not need all three. One valid value stands in for its whole partition.
Boundary value analysis
Bugs cluster at the edges, not the middle. So you test the boundary itself and one step either side: the minimum and one below, the maximum and one above. That is where off-by-one errors live.
Why the two together
Partitions tell you the range behaves. Boundaries tell you the edges behave, and which side broke. Used together a handful of values cover an input space you could never test exhaustively.
Do not forget the wrong type
A number field should also survive empty, letters and symbols. The strongest equivalence sets include the invalid partitions a real user, or an attacker, will reach for.
The invalid cases are not optional
Beginners test that the right values are accepted and stop. The bugs that reach production are usually the invalid ones that were never rejected: the negative price, the 200-character name, the date in the year 3000. Test that the system says no as carefully as you test that it says yes.
This is the floor, not the ceiling
Boundary and equivalence cover one input at a time. When the outcome depends on a combination of inputs, reach for a decision table. When it depends on the order of events, reach for a state model. Master these two first, then layer the rest on top.
Two-value or three-value boundaries
This generator uses the thorough three-value method: the boundary, one step inside, and one step outside each edge. Some teams use the lighter two-value method (the boundary and one step outside only). Three-value catches more off-by-one defects, which is why it is the one worth knowing for an interview. Turn each generated value into a case in the Test Case Builder, and see where boundary and equivalence sit alongside decision tables, state transition and pairwise in the full test design techniques guide.