Gratis program to use my iPad as a second monitor for my Windows computer

Question

I am looking for a gratis program that can allow me to use my iPad as a second monitor for my computer (Microsoft Windows).

I'm aware of:

• Duet Display: costs between 10 USD for the basic version and 20 USD for the "pro" version. Received 17.4k ratings, and the average rating is 4.3/5.
• Air Display: costs around 10 USD and ratings aren't positive.
• iDisplay: costs around 15 USD and ratings aren't positive.
• Splashtop: costs around 5 USD and ratings aren't positive.

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Answer 1

You can use spacedesk:

• gratis for personal use
• requires the computer and the iPad to be in the same network (e.g., same Wi-Fi network)
• also works for Android
• good ratings on Play store and iOS App store but not many ratings.

Answer 2

You can use Splashtop Wired XDisplay HD – Extend & Mirror 4+:

• gratis but might become non-gratis in the future ("**** LIMITED TIME Launch Special (Regular Price: $19.99 USD)" according to the iOS App store)
• works with macOS and Microsoft Windows
• iOS App store ratings: 3.8/5 (186 Ratings)