diff --git a/core/divelist.cpp b/core/divelist.cpp
index 9b3ebf4f3..d198800fc 100644
--- a/core/divelist.cpp
+++ b/core/divelist.cpp
@@ -432,7 +432,7 @@ static void add_dive_to_deco(struct deco_state *ds, const struct dive &dive, boo
 		int j;
 
 		for (j = t0; j < t1; j++) {
-			depth_t depth = { .mm = interpolate(psample.depth.mm, sample.depth.mm, j - t0, t1 - t0) };
+			depth_t depth = interpolate(psample.depth, sample.depth, j - t0, t1 - t0);
 			auto gasmix = loop.at(j).first;
 			add_segment(ds, dive.depth_to_bar(depth), gasmix, 1, sample.setpoint.mbar,
 				    loop_d.at(j), dive.sac,
diff --git a/core/planner.cpp b/core/planner.cpp
index 7489c8c35..69be37dbf 100644
--- a/core/planner.cpp
+++ b/core/planner.cpp
@@ -104,7 +104,7 @@ static void interpolate_transition(struct deco_state *ds, struct dive *dive, dur
 	int32_t j;
 
 	for (j = t0.seconds; j < t1.seconds; j++) {
-		depth_t depth = depth_t { .mm = interpolate(d0.mm, d1.mm, j - t0.seconds, t1.seconds - t0.seconds) };
+		depth_t depth = interpolate(d0, d1, j - t0.seconds, t1.seconds - t0.seconds);
 		add_segment(ds, dive->depth_to_bar(depth), gasmix, 1, po2.mbar, divemode, prefs.bottomsac, true);
 	}
 	if (d1.mm > d0.mm)
diff --git a/core/profile.cpp b/core/profile.cpp
index bd14ac536..7a85dd660 100644
--- a/core/profile.cpp
+++ b/core/profile.cpp
@@ -876,6 +876,7 @@ static void calculate_deco_information(struct deco_state *ds, const struct deco_
 			int time_stepsize = 20;
 			depth_t max_ceiling;
 			depth_t depth { .mm = entry.depth };
+			depth_t prev_depth { .mm = prev.depth };
 
 			divemode_t current_divemode = loop_d.at(entry.sec);
 			struct gasmix gasmix = loop.at(t1).first;
@@ -888,7 +889,7 @@ static void calculate_deco_information(struct deco_state *ds, const struct deco_
 			if (t0 != t1 && t1 - t0 < time_stepsize)
 				time_stepsize = t1 - t0;
 			for (int j = t0 + time_stepsize; j <= t1; j += time_stepsize) {
-				depth_t new_depth { .mm = interpolate(prev.depth, depth.mm, j - t0, t1 - t0) };
+				depth_t new_depth = interpolate(prev_depth, depth, j - t0, t1 - t0);
 				add_segment(ds, dive->depth_to_bar(new_depth),
 					    gasmix, time_stepsize, entry.o2pressure.mbar, current_divemode, entry.sac, in_planner);
 				entry.icd_warning = ds->icd_warning;
@@ -915,7 +916,7 @@ static void calculate_deco_information(struct deco_state *ds, const struct deco_
 				/* If using VPM-B, take first_ceiling_pressure as the deepest ceiling */
 				if (decoMode(in_planner) == VPMB) {
 					if  (current_ceiling.mm >= first_ceiling.mm ||
-					     (time_deep_ceiling == t0 && depth.mm == prev.depth)) {
+					     (time_deep_ceiling == t0 && depth.mm == prev_depth.mm)) {
 						time_deep_ceiling = t1;
 						first_ceiling = current_ceiling;
 						ds->first_ceiling_pressure.mbar = dive->depth_to_mbar(first_ceiling);
diff --git a/core/units.h b/core/units.h
index 0515076df..dc32322d7 100644
--- a/core/units.h
+++ b/core/units.h
@@ -2,6 +2,8 @@
 #ifndef UNITS_H
 #define UNITS_H
 
+#include "interpolate.h"
+
 #include <math.h>
 #ifndef M_PI
 #define M_PI 3.14159265358979323846
@@ -86,6 +88,12 @@
  *	_permille	-> fraction_t in ‰
  *	_percent	-> fraction_t in %
  *
+ * For now, addition and subtraction of values of the same type
+ * are supported.
+ *
+ * A free standing interpolate() function can be used to interpolate
+ * between types of the same kind (see also the interpolate.h header).
+ *
  * We don't actually use these all yet, so maybe they'll change, but
  * I made a number of types as guidelines.
  */
@@ -140,6 +148,15 @@ struct unit_base {
 	}
 };
 
+template <typename T,
+	std::enable_if_t<!std::is_integral<T>::value, bool> = true>
+T interpolate(T a, T b, int part, int whole)
+{
+	return T::from_base(
+		interpolate(a.get_base(), b.get_base(), part, whole)
+	);
+}
+
 struct duration_t : public unit_base<duration_t>
 {
 	int32_t seconds = 0; // durations up to 34 yrs