I would recommend getting a copy of Hindemith's book: Elementary Training for Musicians
In the early chapters he has masses of exercises in sight-reading rhythmic patterns, both as single-line rhythms, and also more complex ones where you have to tap one rhythm while playing another rhythm (very useful for pianists).
The book is also extremely useful for lots of other aspects of musicianship.
I have been delving into the book for years - and still not mastered all it contains (maybe I am just a slow learner!)
music.stackexchange.com/users/2535/Old John
For jazz guitar there is a standard book by William G. Leavitt called Melodic Rhythms for Guitar .
It has lots of examples of so-called "rhythm groups" of increasing complexity.
Playing through this book greatly improves sight-reading of rhythms.
And this is also the clue: there's no shortcut.
You have to sight-read in order to learn it.
In my opinion you shouldn't waste time on looking for some magic tools, just sit down with a piece of music and play.
Of course, it's important to use material that is not beyond your level, but that should be easy to find.
music.stackexchange.com/users/9724/Matt L.
It will depend a little on the instrument.
For example, rhythm guitar with lots of strumming will be slightly different from picking out a melody on, say, a clarinet.
Yes, the rhythmic pattern will be the same, but its execution is different, as is reading chord symbols as opposed to dots.
Tapping a foot is always good, as is internalizing the basic beat with a part of your body.
My violin teacher used to sniff in place of rests, to keep them in time and place.
(Christine Perfect's (McVee) dad, namedropping!)
It worked well, but didn't sound too good.
Metronome will get a mention - sometimes it can be set for twice or thrice the speed to keep more accurate time.
And of course, counting out loud.
1e&a 2e&a et al.
Put the radio etc on and tap along using different patterns, there are literally thousands to choose from, and visualise them as you tap. '
This is 4 quavers, a crotchet, 4 quavers and a crotchet' sort of approach.
The good thing about this is you don't need an instrument - it could even happen as you drive along.
Explaining to the law may be tricky!
music.stackexchange.com/users/2808/Tim
A good idea is to start with the note that is the shortest and make that one beat and count the other notes in relation to the smallest notes.
So for instance if you are in 4/4 time and the shortest notes in the piece is quavers
you make
the quavers one beat, the crotchets 2, minims 4 and semi breves 8.
It is not a perfect method.
You run the risk of either having to count to 8 really fast if the tempo of the piece is brisk or playing the piece much too slow but for a means to teach the basics of rhythm to new players it has merit.
When you get the rhythm down you can much easier learn to play the piece in time and faster later.
music.stackexchange.com/users/7306/Neil Meyer
Just a different sort of flashcard.
Get some blank postcards and a black marker pen and in your best musical handwriting, notate a rhythmic pattern lasting one crotchet on each card.
Each single card could contain for instance (a non-exhaustive list):
crotchet quaver quaver crotchet rest dotted quaver semiquaver semiquaver semiquaver quaver rest Triplet quavers etc.
Create duplicates of each card.
Then shuffle and deal four cards in a row.
Set your metronome and try to read the rhythm, looping around
so you're playing the same bar over and over again.
Pick up the left-hand card and move it to the right hand end, or exchange two cards.
Or replace a card with one from the pack.
You get the idea.
music.stackexchange.com/users/9426/Brian THOMAS
So, there are a number of tools and strategies to learn to sight-read  pitch .  I mean, you just make flashcards and in a while you should be able to read comfortably all notes on all staves, right?  But rhythm is a different beast, isn't it?  If you make flashcards of rhythms, I suppose, you'll memorize those specific rhythms you have made flashcards for.   What is your strategy?  Thank you!
Tools to improve sight-reading of rhythms
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<piano><rhythm><sight-reading>
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music.stackexchange.com/users/19097/Some Dude On The Interwebs
Tools to improve sight-reading of rhythms include Hindemith's book:  Elementary Training for Musicians, Melodic Rhythms for Guitar by William G. Leavitt. Alternatively make flashcards by getting some blank postcards and a black marker pen and in your best musical handwriting, notate a rhythmic pattern lasting one crotchet on each card. The best suggestion will depend on what instrument you are playing, and you probably could still use flashcards to learn rhythms. However, there are also books available to help learn rhythms, such as Hindemith's 'Elementary Training for Musicians or William G. Leavitt's 'Melodic Rhythms'.
There are some books available to help learn rhythms such as Hindemith's 'Elementary Training for Musicians' or William G. Leavitt's 'Melodic Rhythms for Guitar' The best suggestion will probably depend on the instrument in question. You could just try using flashcards for rhythms too.