diff --git a/common/timing.cc b/common/timing.cc
index 225afb5f..47c82a03 100644
--- a/common/timing.cc
+++ b/common/timing.cc
@@ -25,20 +25,72 @@
 
 NEXTPNR_NAMESPACE_BEGIN
 
+static delay_t follow_net(Context *ctx, NetInfo *net, int path_length,
+                          delay_t slack);
+
 // Follow a path, returning budget to annotate
-static delay_t follow_path(Context *ctx, const PortRef &begin, int path_length,
-                           delay_t slack)
+static delay_t follow_user_port(Context *ctx, PortRef &user, int path_length,
+                                delay_t slack)
 {
-    if (ctx->getPortClock(begin.cell, begin.port) != IdString()) {
-        return slack / path_length;
+    delay_t value;
+    if (ctx->getPortClock(user.cell, user.port) != IdString()) {
+        // At the end of a timing path (arguably, should check setup time
+        // here too)
+        value = slack / path_length;
     } else {
-        // ...
+        // Default to the path ending here, if no further paths found
+        value = slack / path_length;
+        // Follow outputs of the user
+        for (auto port : user.cell->ports) {
+            if (port.second.type == PORT_OUT) {
+                delay_t comb_delay;
+                // Look up delay through this path
+                bool is_path = ctx->getCellDelay(user.cell, user.port,
+                                                 port.first, comb_delay);
+                if (is_path) {
+                    NetInfo *net = port.second.net;
+                    if (net) {
+                        delay_t path_budget = follow_net(ctx, net, path_length,
+                                                         slack - comb_delay);
+                        value = std::min(value, path_budget);
+                    }
+                }
+            }
+        }
     }
+
+    if (value < user.budget) {
+        user.budget = value;
+    }
+}
+
+static delay_t follow_net(Context *ctx, NetInfo *net, int path_length,
+                          delay_t slack)
+{
+    delay_t net_budget = slack / path_length;
+    for (auto &usr : net->users) {
+        net_budget = std::min(
+                net_budget, follow_user_port(ctx, usr, path_length + 1, slack));
+    }
+    return net_budget;
 }
 
 void assign_budget(Context *ctx, float default_clock)
 {
-    // TODO
+    for (auto cell : ctx->cells) {
+        for (auto port : cell.second->ports) {
+            if (port.second.type == PORT_OUT) {
+                IdString clock_domain =
+                        ctx->getPortClock(cell.second, port.first);
+                if (clock_domain != IdString()) {
+                    delay_t slack = delay_t(
+                            1.0e12 / default_clock); // TODO: clock constraints
+                    if (port.second.net)
+                        follow_net(ctx, port.second.net, 0, slack);
+                }
+            }
+        }
+    }
 }
 
 NEXTPNR_NAMESPACE_END
diff --git a/dummy/arch.cc b/dummy/arch.cc
index 64841374..fa0d8bd3 100644
--- a/dummy/arch.cc
+++ b/dummy/arch.cc
@@ -178,10 +178,10 @@ std::vector<GraphicElement> Arch::getPipGraphics(PipId pip) const
 
 // ---------------------------------------------------------------
 
-delay_t Arch::getCellDelay(const CellInfo *cell, IdString fromPort,
-                           IdString toPort) const
+bool Arch::getCellDelay(const CellInfo *cell, IdString fromPort,
+                        IdString toPort, delay_t &delay) const
 {
-    return 0;
+    return false;
 }
 
 IdString Arch::getPortClock(const CellInfo *cell, IdString port) const
diff --git a/dummy/arch.h b/dummy/arch.h
index 61a09e00..39c48232 100644
--- a/dummy/arch.h
+++ b/dummy/arch.h
@@ -134,8 +134,8 @@ struct Arch : BaseCtx
     std::unordered_set<WireId> wireGraphicsReload;
     std::unordered_set<PipId> pipGraphicsReload;
 
-    delay_t getCellDelay(const CellInfo *cell, IdString fromPort,
-                         IdString toPort) const;
+    bool getCellDelay(const CellInfo *cell, IdString fromPort, IdString toPort,
+                      delay_t &delay) const;
     IdString getPortClock(const CellInfo *cell, IdString port) const;
     bool isClockPort(const CellInfo *cell, IdString port) const;
 };
diff --git a/ice40/arch.cc b/ice40/arch.cc
index 380f3386..7baaef8d 100644
--- a/ice40/arch.cc
+++ b/ice40/arch.cc
@@ -418,11 +418,11 @@ std::vector<GraphicElement> Arch::getPipGraphics(PipId pip) const
 
 // -----------------------------------------------------------------------
 
-delay_t Arch::getCellDelay(const CellInfo *cell, IdString fromPort,
-                           IdString toPort) const
+bool Arch::getCellDelay(const CellInfo *cell, IdString fromPort,
+                        IdString toPort, delay_t &delay) const
 {
     // TODO
-    return 0;
+    return false;
 }
 
 IdString Arch::getPortClock(const CellInfo *cell, IdString port) const
diff --git a/ice40/arch.h b/ice40/arch.h
index 172541c0..356f88da 100644
--- a/ice40/arch.h
+++ b/ice40/arch.h
@@ -758,9 +758,10 @@ struct Arch : BaseCtx
 
     // -------------------------------------------------
 
-    // Get the delay through a cell from one port to another
-    delay_t getCellDelay(const CellInfo *cell, IdString fromPort,
-                         IdString toPort) const;
+    // Get the delay through a cell from one port to another, returning false
+    // if no path exists
+    bool getCellDelay(const CellInfo *cell, IdString fromPort, IdString toPort,
+                      delay_t &delay) const;
     // Get the associated clock to a port, or empty if the port is combinational
     IdString getPortClock(const CellInfo *cell, IdString port) const;
     // Return true if a port is a clock