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Septic Calculations

Most residential housing in rural areas of southern California uses a septic system. The lengths of sewer needed to service sparsely populated areas are just not cost effective. The calculator given here is the one used in San Diego County to determine the area needed for a septic system.

A traditional septic system consists of a series of trenches which comprise a 'leach field'. The length of the trenches in the field is calculated according to the rate at which water percolates through the soil, The other parameter is the number of bedrooms in the proposed dwelling.
DISPOSAL TRENCH LENGTH BASED ON PERCOLATION TEST RESULTS
 
Percolation Rate Average     Percolation Rate Average
Time in Minutes/Inch (MPI)    Time in Minutes/Inch (MPI) 
Number of Bedrooms     Number of Bedrooms
         
MPI 1* 2 3 4 5 6 MPI 1* 2 3 4 5
1 200 200 240 270 280 300 41 310 385 460 520 575
2 200 200 240 270 280 300 42 310 390 470 530 585
3 200 200 240 270 280 300 43 310 390 470 530 585
4 200 220 260 290 300 310 44 310 395 480 540 595
5 200 240 290 320 320 340 45 320 400 480 540 595
6 200 250 300 340 350 360 46 320 400 480 540 595
7 210 260 310 350 370 380 47 320 405 490 550 605
8 210 265 320 360 390 400 48 330 410 490 550 605
9 220 270 320 360 400 410 49 330 410 500 560 615
10 220 275 330 370 410 420 50 330 415 500 560 615
11 220 280 340 380 420 430 51 340 420 500 560 615
12 230 285 340 380 430 440 52 340 420 510 570 625
13 230 290 350 390 430 450 53 340 425 510 580 635
14 235 295 350 400 440 460 54 340 430 520 580 635
15 240 300 360 400 450 470 55 340 430 520 580 635
16 240 300 360 410 450 490 56 350 435 520 590 645
17 240 305 370 410 460 500 57 350 440 530 590 645
18 250 310 370 420 460 510 58 350 440 530 600 655
19 250 310 380 420 470 520 59 350 445 540 600 655
20 250 315 380 430 470 520 60 360 450 540 610 665
21 260 320 380 430 480 530 61 370 460 550 620 690
22 260 320 390 440 480 530 62 380 470 560 630 680
23 260 325 390 440 490 550 63 390 480 570 640 690
24 260 330 400 450 500 560 64 400 490 580 650 700
25 260 330 400 450 500 560 65 420 500 580 660 710
26 270 335 400 450 510 570 66 420 510 600 670 720
27 270 340 410 460 515 575 67 430 520 610 680 730
28 270 340 410 460 515 575 68 440 530 620 690 740
29 270 345 420 470 525 585 69 450 540 630 700 750
30 280 350 420 470 525 585 70 460 550 640 710 760
31 280 350 420 480 535 595 71 470 560 650 720 770
32 280 355 430 480 535 595 72 480 570 660 730 780
33 290 360 430 490 545 605 73 490 580 670 740 790
34 290 360 440 490 545 605 74 500 590 680 750 800
35 290 365 440 500 555 615 75 510 600 690 760 810
36 300 370 440 500 555 615 76 520 610 700 770 820
37 300 370 450 500 555 615 77 530 620 710 780 830
38 300 375 450 510 565 625 78 540 630 720 790 840
39 300 380 460 510 565 625 79 550 640 730 800 850
40 300 380 460 520 575 635 80 560 650 740 810 860
Percolation Rate Average
Time  in Minutes/Inch (MPI)
Number of Bedrooms
MPI 2 3 4 5 6
81 660 750 820 870 910
82 670 760 830 880 920
83 680 770 840 890 930
84 690 780 850 900 940
85 700 790 860 910 950
86 710 800 870 920 960
87 720 810 880 930 970
88 730 820 890 940 980
89 740 830 900 950 990
90 755 845 915 965 1005
91 770 860 930 980 1020
92 785 875 945 995 1035
93 800 890 960 1010 1050
94 815 905 975 1025 1065
95 830 920 990 1040 1080
96 845 935 1005 1055 1095
97 860 950 1020 1070 1110
98 875 965 1035 1085 1125
99 890 980 1050 1100 1140
100 905 995 1065 1115 1155
101 920 1010 1080 1130 1170
102 935 1025 1095 1145 1185
103 950 1040 1110 1160 1200
104 965 1055 1125 1175 1215
105 980 1070 1140 1190 1230
106 995 1085 1155 1205 1245
107 1010 1100 1170 1220 1260
108 1025 1115 1185 1230 1270
109 1040 1130 1200 1250 1290
110 1055 1145 1215 1265 1305
111 1070 1160 1230 1280 1320
112 1085 1175 1245 1295 1335
113 1100 1190 1260 1310 1350
114 1115 1205 1275 1325 1365
115 1130 1220 1290 1340 1380
116 1145 1235 1305