Start with a full chapter, free.

What does a free account actually get me?

The first chapter of your exam in full: every problem, every worked solution, and Castor on every one. It never asks for a card and it never expires. Pro opens every other chapter, the diagnostic and full-length mock exams.

Can I cancel?

Anytime. Pro renews monthly until you cancel — cancel whenever and you keep access through the month you've already paid for, then you're never charged again. Your account doesn't close: it returns to the free tier — your practice history stays, and your mock reports are kept and reopen if you come back to Pro.

Is there a pass guarantee?

No. We'd rather you judge the product than a promise. The free chapter lets you judge the problems and Castor before you pay a cent, and Pro is cancel-anytime — the proof is the practice itself, not a guarantee.

Do you do refunds?

Payments aren't refunded once a billing period starts. Cancelling stops the next charge rather than reversing the last one, and there is no lock-in. (If something's wrong on our end — a double charge, or a renewal you didn't expect — email us and we'll fix it.)

Which exams?

PE Mechanical (HVAC & Refrigeration, Machine Design & Materials, and Thermal & Fluid Systems), PE Electrical: Power, PE Civil: Water Resources & Environmental and PE Civil: Transportation.

Can I expense this to my employer?

Most engineering firms reimburse PE prep. Every Pro month comes with a Stripe receipt by email the moment you're charged — one clean line-item each, ready to forward.Copy an email template for your manager →

Is my AI usage limited?

There is no meter on either plan: on the free tier Castor is on every problem in your free chapter, and on Pro on every problem. A quiet fair-use cap exists to stop abuse, never normal prep.

More questions? Get in touch, or start with a free chapter.