diff --git a/src/quic_cc_cubic.c b/src/quic_cc_cubic.c
index 51d11b11c..fc21aa0f5 100644
--- a/src/quic_cc_cubic.c
+++ b/src/quic_cc_cubic.c
@@ -243,13 +243,24 @@ static inline void quic_cubic_update(struct quic_cc *cc, uint32_t acked)
 			c->W_target = path->cwnd;
 		}
 		else {
+			uint64_t wnd_diff;
+
 			/* K value computing (in seconds):
 			 * K = cubic_root((W_max - cwnd_epoch)/C) (Figure 2)
-			 * Note that K is stored in milliseconds.
+			 * Note that K is stored in milliseconds and that
+			 * 8000 * 125000 = 1000^3.
+			 *
+			 * Supporting 2^40 windows, shifted by 10, leaves ~13 bits of unused
+			 * precision. We exploit this precision for our NS conversion by
+			 * multiplying by 8000 without overflowing, then later by 125000
+			 * after the divide so that we limit the precision loss to the minimum
+			 * before the cubic_root() call."
 			 */
-			c->K = cubic_root(((c->last_w_max - path->cwnd) << CUBIC_SCALE_FACTOR_SHIFT) / (CUBIC_C_SCALED * path->mtu));
-			/* Convert to milliseconds. */
-			c->K *= 1000;
+			wnd_diff = (c->last_w_max - path->cwnd) << CUBIC_SCALE_FACTOR_SHIFT;
+			wnd_diff *= 8000ULL;
+			wnd_diff /= CUBIC_C_SCALED * path->mtu;
+			wnd_diff *= 125000ULL;
+			c->K = cubic_root(wnd_diff);
 			c->W_target = c->last_w_max;
 		}