diff --git a/src/core/nanoev.ml b/src/core/nanoev.ml
index c15935e..75d34b7 100644
--- a/src/core/nanoev.ml
+++ b/src/core/nanoev.ml
@@ -7,6 +7,7 @@ module Impl = struct
     clear: 'st -> unit;
     wakeup_from_outside: 'st -> unit;
     close: 'st -> Unix.file_descr -> unit;
+    max_fds: 'st -> int;
     on_readable:
       'a 'b.
       'st ->
@@ -39,6 +40,7 @@ type t = Impl.t
 let[@inline] clear (Ev (ops, st)) = ops.clear st
 let[@inline] wakeup_from_outside (Ev (ops, st)) = ops.wakeup_from_outside st
 let[@inline] close (Ev (ops, st)) fd = ops.close st fd
+let[@inline] max_fds (Ev (ops, st)) = ops.max_fds st
 
 let[@inline] on_readable (Ev (ops, st)) fd x y f : unit =
   ops.on_readable st fd x y f
diff --git a/src/core/nanoev.mli b/src/core/nanoev.mli
index 4792cd6..213b9de 100644
--- a/src/core/nanoev.mli
+++ b/src/core/nanoev.mli
@@ -9,6 +9,7 @@ module Impl : sig
     clear: 'st -> unit;
     wakeup_from_outside: 'st -> unit;
     close: 'st -> Unix.file_descr -> unit;
+    max_fds: 'st -> int;
     on_readable:
       'a 'b.
       'st ->
@@ -43,6 +44,9 @@ val step : t -> unit
 val close : t -> Unix.file_descr -> unit
 (** Close the file descriptor and clean it up *)
 
+val max_fds : t -> int
+(** Maximum number of file descriptors that can be observed at once. *)
+
 val on_readable :
   t -> Unix.file_descr -> 'a -> 'b -> (closed:bool -> 'a -> 'b -> unit) -> unit
 
diff --git a/src/unix/nanoev_unix.ml b/src/unix/nanoev_unix.ml
index 7d656c1..b74ea06 100644
--- a/src/unix/nanoev_unix.ml
+++ b/src/unix/nanoev_unix.ml
@@ -243,12 +243,16 @@ let step (self : st) : unit =
 
   ()
 
+(* limit for select is fixed and known *)
+let max_fds _ = 1024
+
 let ops : st Nanoev.Impl.ops =
   {
     step;
     close;
     on_readable;
     on_writable;
+    max_fds;
     run_after_s;
     wakeup_from_outside;
     clear;