An online WinISD alternative for car subwoofer boxes
Updated October 13, 2026
WinISD is the classic free program for modeling speaker boxes, and it is still a great tool. Sub Box Calculator covers the same modeling and adds what a car-audio build needs next: the car, the drawings and the cut list — in a browser, with nothing to install.
Same physics
Both model the subwoofer from its Thiele/Small parameters: sealed and ported boxes, response, cone excursion, port air speed, group delay and impedance. The port length uses the same end correction as WinISD (0.732 × diameter for a round port), and we check our results against WinISD for a set of real drivers when we change the model.
What is different
| WinISD | Sub Box Calculator | |
|---|---|---|
| Runs on | Windows (installed) | Any browser: Windows, Mac, Linux, phone |
| Driver data | Enter or import parameters | Database of about 2,000 car subwoofers, or enter your own |
| Car cabin gain | Not modeled | Modeled by vehicle type and placement |
| Box dimensions | Volume only | Fits width, height and depth to your space, including wedge boxes |
| Port | Length for a given area | Round or slot, folded slot drawn to fit inside the box |
| Plans | — | Panel drawings, cut list, sheet layout, PDF report, DXF for CNC |
| Units | Metric or imperial | Inches/ft³ or mm/liters |
When to use which
Use WinISD if you are designing home speakers with passive filters or want its specialized enclosure types. Use Sub Box Calculator when you are building a car subwoofer box and want to go from the driver to a cut list in one place — or when you are not on Windows.
Try it
Open the calculator, pick your sub from the database, and the model, the drawings and the cut list are ready in a minute. It is free and needs no account.